Fixed unknown translation key. (#16057)

Author: KN4CK3R

templates/repo/diff/comment_form.tmpl

@@ -22,7 +22,7 @@
 			</div>
 		</div>
 		<div class="field footer mx-3">
-			<span class="markup-info">{{svg "octicon-markup"}} {{$.root.i18n.Tr "repo.diff.comment.markup_info"}}</span>
+			<span class="markup-info">{{svg "octicon-markup"}} {{$.root.i18n.Tr "repo.diff.comment.markdown_info"}}</span>
 			<div class="ui right">
 				{{if $.reply}}
 					<button class="ui submit green tiny button btn-reply" type="submit">{{$.root.i18n.Tr "repo.diff.comment.reply"}}</button>