Update themes.rst

Changed references of Plugin to plugins
I can update that as well. I missed it.

ADmad wrote:
>
> In en/views/themes.rst:
>
> > @@ -19,14 +19,14 @@ You can also set or change the theme name within 
> an action or within the
> > $this->theme ='AnotherExample';
> >
> > Theme view files need to be within a plugin with the same name. For 
> example,
> > -the above theme would be found in ``/Plugin/AnotherExample/Template``.
> > +the above theme would be found in ``/plugins/AnotherExample/Template``.
>
> The path should be |/plugins/AnotherExample/src/Template|. Similarly
> all other plugin template paths in this patch are missing |/src/|.
>
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> <https://github.com/cakephp/docs/pull/1489/files#r14979918>.