Googletest export

Update the `DescribeTo` signature in the code example at the Writing New Polymorphic Matchers section in gmock_cook_book.

`os` should be a pointer to be consistent with the implementation, which dereference it as `*os`.

PiperOrigin-RevId: 370693387

docs/gmock_cook_book.md

@@ -3696,7 +3696,7 @@ class NotNullMatcher {
   }
 
   // Describes the property of a value matching this matcher.
-  void DescribeTo(std::ostream& os) const { *os << "is not NULL"; }
+  void DescribeTo(std::ostream* os) const { *os << "is not NULL"; }
 
   // Describes the property of a value NOT matching this matcher.
   void DescribeNegationTo(std::ostream* os) const { *os << "is NULL"; }