[docs] Update path to clang-tools-extra

Summary:
> tools/clang/tools/extra
has become
>clang-tools-extra
which was not updated in all docs.

Reviewers: alexfh, aaron.ballman, ilya-biryukov, juliehockett

Reviewed By: aaron.ballman

Subscribers: Jim, cfe-commits

Tags: #clang-tools-extra, #clang

Differential Revision: https://reviews.llvm.org/D71982