Update link in development documentation

The old link was a 404 page.

HACKING

@@ -498,7 +498,7 @@ first (scintilla or ctags).
 If you want to reuse an existing lexer and/or tag parser, making a
 custom filetype is probably easier - it doesn't require any
 changes to the source code. Follow instructions in the manual:
-http://geany.org/manual/geany.html#custom-filetypes. Don't forget to
+http://geany.org/manual/index.html#custom-filetypes. Don't forget to
 update the ``[Groups]`` section in ``filetype_extensions.conf``.
 
 .. warning::