doc: fix link to DoubleEndedIterator in std::string::String docs.

[matthiaskrgr]

No description provided.

[rust-highfive]

r? @Kimundi

(rust_highfive has picked a reviewer for you, use r? to override)

[rust-highfive]

The job x86_64-gnu-llvm-5.0 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

travis_time:end:2186c604:start=1543833366901124773,finish=1543833368046465512,duration=1145340739
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout

Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#Pull-Requests-and-Security-Restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
$ export SCCACHE_REGION=us-west-1
Setting environment variables from .travis.yml
$ export IMAGE=x86_64-gnu-llvm-5.0
---
travis_time:start:test_codegen
Check compiletest suite=codegen mode=codegen (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
[00:54:25] 
[00:54:25] running 119 tests
[00:54:28] i..ii...iii..iiii.....i...i.........i..iii.............i.....i.....ii...i..i.ii..............i...ii. 100/119
[00:54:29] .ii.i.....iiii.....
[00:54:29] 
[00:54:29]  finished in 3.367
[00:54:29] travis_fold:end:test_codegen

---
travis_time:start:test_debuginfo
Check compiletest suite=debuginfo mode=debuginfo-both (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
[00:54:43] 
[00:54:43] running 118 tests
[00:55:06] .iiiii...i.....i..i...i..i.i..i.i..i.....i..i....i..........iiii.........i.i....i...i.......ii.i.i.i 100/118
[00:55:10] ......iii.i.....ii
[00:55:10] 
[00:55:10]  finished in 27.202
[00:55:10] travis_fold:end:test_debuginfo

---
[01:20:11] travis_fold:end:stage0-linkchecker

[01:20:11] travis_time:end:stage0-linkchecker:start=1543838186664567473,finish=1543838189096460201,duration=2431892728

[01:20:11] std/primitive.str.html:585: broken link - iter/trait.DoubleEndedIterator.html
[01:20:11] std/primitive.str.html:663: broken link - iter/trait.DoubleEndedIterator.html
[01:20:11] std/primitive.str.html:695: broken link - iter/trait.DoubleEndedIterator.html
[01:20:11] std/primitive.str.html:796: broken link - iter/trait.DoubleEndedIterator.html
[01:20:11] std/primitive.str.html:816: broken link - iter/trait.DoubleEndedIterator.html
[01:20:11] std/primitive.str.html:835: broken link - iter/trait.DoubleEndedIterator.html
[01:20:11] std/primitive.str.html:860: broken link - iter/trait.DoubleEndedIterator.html
[01:20:16] thread 'main' panicked at 'found some broken links', src/tools/linkchecker/main.rs:49:9
[01:20:16] 
[01:20:16] 
[01:20:16] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/linkchecker" "/checkout/obj/build/x86_64-unknown-linux-gnu/doc"
[01:20:16] expected success, got: exit code: 101
[01:20:16] expected success, got: exit code: 101
[01:20:16] 
[01:20:16] 
[01:20:16] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test
[01:20:16] Build completed unsuccessfully in 0:36:15
[01:20:16] make: *** [check] Error 1
[01:20:16] Makefile:58: recipe for target 'check' failed
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:0005d1cb
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Mon Dec  3 11:56:35 UTC 2018
---
travis_time:end:26c68928:start=1543838197544871556,finish=1543838197643107783,duration=98236227
travis_fold:end:after_failure.4
travis_fold:start:after_failure.5
travis_time:start:05ae65b0
$ cat ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers || true
cat: ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers: No such file or directory
travis_fold:end:after_failure.5
travis_fold:start:after_failure.6
travis_time:start:053a44b8
$ dmesg | grep -i kill

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[matthiaskrgr]

Not sure what CI is talking about.. I'll just make a ticket.