GenerateExportHeader: Fix name of example function definition

The usage example declares a deprecated internal function but defines it
under a different name, so the example would not link.

Modules/GenerateExportHeader.cmake

@@ -356,7 +356,7 @@ And in the ABI header files:
     std::cout << "legacyPublicFunction() is deprecated" << std::endl;
   }
 
-  void internalLegacyFunction()
+  void legacyInternalFunction()
   {
     std::cout << "legacyInternalFunction() is deprecated" << std::endl;
   }