grammar improvements

Add 3 missing "the".

CONTRIBUTING.md

@@ -4,12 +4,12 @@
 
 Before sending your pull requests, make sure you followed this list.
 
-- Read [contributing guidelines](CONTRIBUTING.md).
-- Read [Code of Conduct](CODE_OF_CONDUCT.md).
+- Read the [contributing guidelines](CONTRIBUTING.md).
+- Read the [Code of Conduct](CODE_OF_CONDUCT.md).
 - Ensure you have signed the [Contributor License Agreement (CLA)](https://cla.developers.google.com/).
 - Check if your changes are consistent with the [guidelines](https://github.com/tensorflow/tensorflow/blob/master/CONTRIBUTING.md#general-guidelines-and-philosophy-for-contribution).
 - Changes are consistent with the [Coding Style](https://github.com/tensorflow/tensorflow/blob/master/CONTRIBUTING.md#c-coding-style).
-- Run [Unit Tests](https://github.com/tensorflow/tensorflow/blob/master/CONTRIBUTING.md#running-unit-tests).
+- Run the [unit tests](https://github.com/tensorflow/tensorflow/blob/master/CONTRIBUTING.md#running-unit-tests).
 
 ## How to become a contributor and submit your own code