testing: minor typo in documentation

[t-yuki]

https://code.google.com/p/go/source/browse/src/testing/testing.go?r=804cc55d6b47e49e4bf46baa4ca9ec44b9684bed#47
It says `The benchmark package will vary b.N`... but there are no `benchmark` package
currently.
It should be `The package will vary b.N` with taking account of the description in
Example section. In this context, `the package` indicates testing package.

[ianlancetaylor]

Comment 1:

Labels changed: added repo-main, release-none, documentation.

[gopherbot]

Comment 2:

CL https://golang.org/cl/146680043 mentions this issue.

[griesemer]

Comment 3:

This issue was closed by revision 5edff32.

Status changed to Fixed.