Fixing typo in documentation.

This upstreams a Google-internal change.

googletest/include/gtest/gtest.h

@@ -1287,7 +1287,7 @@ class GTEST_API_ UnitTest {
   internal::UnitTestImpl* impl() { return impl_; }
   const internal::UnitTestImpl* impl() const { return impl_; }
 
-  // These classes and funcions are friends as they need to access private
+  // These classes and functions are friends as they need to access private
   // members of UnitTest.
   friend class Test;
   friend class internal::AssertHelper;