Update docs to mention .inc.php for roundcube

A recent patch updated the roundcube overrides to use .inc.php vs .inc,
as it's done in roundcube (and as suggested by roundcube plugin docs).

It corrected the overrides and fixed it's section in the faq, but missed
a section in the configuration docs that to the overrides. This fixes
that oversight.

docs/configuration.rst

@@ -332,7 +332,7 @@ If ``ROUNDCUBE_PLUGINS`` is not set the following plugins are enabled by default
 
 To disable all plugins just set ``ROUNDCUBE_PLUGINS`` to ``mailu``.
 
-To configure a plugin add php files named ``*.inc`` to roundcube's :ref:`override section <override-label>`.
+To configure a plugin add php files named ``*.inc.php`` to roundcube's :ref:`override section <override-label>`.
 
 Mail log settings
 -----------------