documentation, change mater branch to main

docs/samples.md

 # Googletest Samples
 
 If you're like us, you'd like to look at
-[googletest samples.](https://github.com/google/googletest/tree/master/googletest/samples)
+[googletest samples.](https://github.com/google/googletest/tree/main/googletest/samples)
 The sample directory has a number of well-commented samples showing how to use a
 variety of googletest features.