Fix Translator comment

Fixes the translation comment (TO TRANSLATORS:) to be part of the English string:

#: src/appendix/contributing.xml:266(para)
msgid ""
"Don't include English text in images. Translators can't translate it and "
"many users don't like that. Use XML captions instead, or provide a .xcf file "
"in the <menuchoice><guimenu>Docs</guimenu><guisubmenu>xcf images</"
"guisubmenu></menuchoice> folder, indicating it by a comment in the XML file: "
"<markup>&lt;!--TO TRANSLATORS: Corresponding .xcf file is in https://gitlab."
"gnome.org/GNOME/gimp-help/tree/master/docs/xcf%20images --&gt;</markup>"
msgstr ""