Fix typos in sample comments

Closes https://github.com/wxWidgets/wxWidgets/pull/2513

samples/xrc/xrcdemo.cpp

@@ -102,7 +102,7 @@ bool MyApp::OnInit()
 
     // Load all of the XRC files that will be used. You can put everything
     // into one giant XRC file if you wanted, but then they become more
-    // diffcult to manage, and harder to reuse in later projects.
+    // difficult to manage, and harder to reuse in later projects.
     if ( !wxXmlResource::Get()->LoadAllFiles("rc") )
         return false;