Update README.md with proper clang flag for install.sh

Following the instruction in the README for OS X revealed a minor documentation error.
```
➜  YouCompleteMe git:(master) ✗ ./install.sh --clang-completer --system-clang --system-boost
Usage: /Users/jwomack/.vim/bundle/YouCompleteMe/third_party/ycmd/build.sh [--clang-completer [--system-libclang]] [--system-boost] [--omnisharp-completer]

➜  YouCompleteMe git:(master) ✗ ./install.sh --clang-completer --system-libclang --system-boost
/var/folders/b3/2bw5vyb14s32ktj5glp_r1ydcc9ghq/T/ycm_build.XXXXXX.X7SooY2d ~/.vim/bundle/YouCompleteMe
```