message: Fix args to update-alternatives.
diff: @@ -236,11 +236,14 @@ $ CC=/usr/bin/clang CXX=/usr/bin/clang++ \
  ```
  You can also export the `CC` and `CXX` environment variables to always point to `clang`. Another
  -solution is to use `update-alternatives` to change the default compiler:
  +solution is to use `update-alternatives` to both change the default compiler, and point to a
  +specific version of clang:
  ```
  -$ update-alternatives --install /usr/bin/cc /usr/bin/clang 100
  -$ update-alternatives --install /usr/bin/c++ /usr/bin/clang++ 100
  +$ update-alternatives --install /usr/bin/cc cc /usr/bin/clang 100
  +$ update-alternatives --install /usr/bin/c++ c++ /usr/bin/clang++ 100
  +$ update-alternatives --install /usr/bin/clang clang /usr/bin/clang-5.0 100
  +$ update-alternatives --install /usr/bin/clang++ clang++ /usr/bin/clang++-5.0 100
  ```
  Finally, invoke `ninja`:
  