Changes the googlemock link

[s-nil]

Summary

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

[labath]

Simple enough. Do you have commit access or do you need me to do that?

[s-nil]

No, I do not have commit access. Would you do the commit?

[labath]

Done with 935729e4c63a0. Thanks for the fix.

llvm/docs/TestingGuide.rst

 historical reasons, these tests are also referred to as the "nightly
 tests" in places, which is less ambiguous than "test-suite" and remains
 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/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
 ----------------
 
 The regression tests are small pieces of code that test a specific
 feature of LLVM or trigger a specific bug in LLVM. The language they are
 written in depends on the part of LLVM being tested. These tests are driven by