Fix comment plugin typo

plugins/tiddlywiki/comments/header-view-template-segment.tid

@@ -19,7 +19,7 @@ list-before: $:/core/ui/ViewTemplate/body
 <$list filter="[all[current]role[comment]]" variable="ignore">
 <div class="tc-is-comment-header">
 <p>
-This tiddler a comment on
+This tiddler is a comment on
 <$list filter="[list<currentTiddler>sort[title]]">
 <<find-original-comment>>&nbsp;
 </$list>