Inappropriate handling of environment variable LD_LIBRARY_PATH in README.

[taro-yamada]

I'm sorry, this is a very trivial thing.

README.md said

If you get an error like error while loading shared libraries while starting up rustfmt you should try the following:
On Linux:
export LD_LIBRARY_PATH=$(rustc --print sysroot)/lib:$LD_LIBRARY_PATH

In this case, if LD_LIBRARY_PATH is empty, the current directory will be included in LD_LIBRARY_PATH and causes potential security problems.

I think that it is good to do as following.
export LD_LIBRARY_PATH=$(rustc --print sysroot)/lib${LD_LIBRARY_PATH:+:${LD_LIBRARY_PATH}}

[topecongiro]

Thanks! I think we can totally remove this part from README as now rustfmt does not depend on libsyntax directly anymore.

[topecongiro]

Closed via #2974.