[Docs] Update googletest docs link.

The documentation link of Google Test on GitHub have been moved to the
top-level docs directory.
Thus, the original link is invalid now.

Reviewed By: Pavel Labath

Differential Revision: https://reviews.llvm.org/D99559

llvm/docs/TestingGuide.rst

@@ -44,8 +44,8 @@ in use although we run them much more often than nightly.
 Unit tests
 ----------
 
-Unit tests are written using `Google Test <https://github.com/google/googletest/blob/master/googletest/docs/primer.md>`_
-and `Google Mock <https://github.com/google/googletest/blob/master/googlemock/docs/for_dummies.md>`_
+Unit tests are written using `Google Test <https://github.com/google/googletest/blob/master/docs/primer.md>`_
+and `Google Mock <https://github.com/google/googletest/blob/master/docs/gmock_for_dummies.md>`_
 and are located in the ``llvm/unittests`` directory.
 In general unit tests are reserved for targeting the support library and other
 generic data structure, we prefer relying on regression tests for testing