Footnote documentation - add example

The current example didn't contain a footnote to test. Anyone copying and pasting the example would have been confused (I know I was!)

docs/2.5/extensions/footnotes.md

@@ -85,7 +85,7 @@ $environment->addExtension(new FootnoteExtension());
 
 // Instantiate the converter engine and start converting some Markdown!
 $converter = new MarkdownConverter($environment);
-echo $converter->convert('# Hello World!');
+echo $converter->convert("This is a footnote[^test] test.\n\n[^test]: Doesn't it look good!");
 ```
 
 ## Configuration