Fix syntax error in interface headers

Although these headers are not supposed to be compiled, it's still
better to avoid at least the obvious typos in them.

Closes https://github.com/wxWidgets/wxWidgets/pull/2140

interface/wx/richtext/richtextprint.h

@@ -173,7 +173,7 @@ public:
     /**
         Assignment operator.
     */
-    void operator operator=(const wxRichTextHeaderFooterData& data);
+    void operator=(const wxRichTextHeaderFooterData& data);
 };