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.

interface/wx/cshelp.h

@@ -182,7 +182,7 @@ public:
       so that this button is only added to a dialog for non-Windows platforms
       (use wxDIALOG_EX_CONTEXTHELP on Windows).
 
-    Note that on OS X, the cursor does not change when in context-sensitive
+    Note that on macOS, the cursor does not change when in context-sensitive
     help mode.
 
     @library{wxcore}