Change documentation references from OS X to macOS (#1927)

Since OS X 10.12 it has been named macOS so it makes sense
to reference it in documentation as such, even when it
sometimes refers to older versions which were called (Mac) OS X.

docs/doxygen/mainpages/samples.h

@@ -652,7 +652,7 @@ first argument of @c save, @c load or @c delete, followed by the "service" and
 "user" arguments as used by wxSecretStore methods. After storing some password,
 you can check that it can be retrieved later and also that it can be seen in
 the OS-provided password manager (e.g. credential manager under MSW or keychain
-utility under OS X).
+utility under macOS).
 
 @sampledir{secretstore}