Update CONTRIBUTING.md (tensorflow#19794)

Just update clang-tidy to `clang-tidy`.

Author: tucan9389

CONTRIBUTING.md

@@ -90,7 +90,7 @@ Bazel BUILD files also need to include a license section, e.g.,
 Changes to TensorFlow C++ code should conform to
 [Google C++ Style Guide](https://google.github.io/styleguide/cppguide.html).
 
-Use `clang-tidy` to check your C/C++ changes. To install clang-tidy on ubuntu:16.04, do:
+Use `clang-tidy` to check your C/C++ changes. To install `clang-tidy` on ubuntu:16.04, do:
 
 ```bash
 apt-get install -y clang-tidy