Change the googlemock link

Summary:
This commit changes the googlemock link in the Testing Guide doc as googlemock is absorbed into the GoogleTest project

Reviewers: shafik, labath, JDevlieghere, mib

Reviewed By: labath

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73082

Author: s-nil

llvm/docs/TestingGuide.rst

@@ -45,7 +45,7 @@ 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/ForDummies.md>`_
+and `Google Mock <https://github.com/google/googletest/blob/master/googlemock/docs/for_dummies.md>`_
 and are located in the ``llvm/unittests`` directory.
 
 Regression tests