Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -80,7 +80,7 @@ fairly rigid coding style, as defined by the
 [google-styleguide](https://github.com/google/styleguide) project. All patches
 will be expected to conform to the style outlined
 [here](https://google.github.io/styleguide/cppguide.html). Use
-[.clang-format](https://github.com/google/googletest/blob/master/.clang-format)
+[.clang-format](https://github.com/google/googletest/blob/main/.clang-format)
 to check your formatting.
 
 ## Requirements for Contributors