Bug 22660: (follow-up) Re-word system preference description

Bug 22660: (follow-up) Re-word system preference description

This patch rewords the system preference description to focus on the
generic description of the editor rather than the name of the plugin:

    tinymce: "a WYSIWYG editor (TinyMCE)"
    codemirror: "a text editor (CodeMirror)"

Signed-off-by: Lisette Scheer <lisetteslatah@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit 23846781751d7fb808b3fa4b857da72706653575)

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>