Fix example code in wxArtProvider docs

Add a missing ampersand to the third parameter of overridden
CreateBitmap(), to make it clear it is a reference.

Also add semicolons after methods declarations.

Closes https://github.com/wxWidgets/wxWidgets/pull/2299

Closes #19125.

interface/wx/artprov.h

@@ -112,11 +112,11 @@ const char* wxART_EDIT;
       protected:
         wxBitmap CreateBitmap(const wxArtID& id,
                               const wxArtClient& client,
-                              const wxSize size)
+                              const wxSize& size);
 
         // optionally override this one as well
         wxIconBundle CreateIconBundle(const wxArtID& id,
-                                      const wxArtClient& client)
+                                      const wxArtClient& client);
         { ... }
       };
       ...