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/bookctrl.h

@@ -12,7 +12,7 @@
     bits is set if point is over a tab.
     Notice that wxOSX currently only returns wxBK_HITTEST_ONLABEL or
     wxBK_HITTEST_NOWHERE and never the other values, so you should only test
-    for these two in the code that should be portable under OS X.
+    for these two in the code that should be portable under macOS.
  */
 enum
 {