[css-inline] fixed a stray non-plural initial-letter

Author: dauwhe

css-inline-3/Overview.bs

@@ -1397,7 +1397,7 @@ Edge Effects: Indentation and Hanging Punctuation</h4>
 		<figcaption>Initial letter with text indent. </figcaption>
 	</figure>
 
-	ISSUE: The interaction of 'initial-letter' and 'hanging-punctuation'
+	ISSUE: The interaction of 'initial-letters' and 'hanging-punctuation'
 	is <a href="hhttps://github.com/w3c/csswg-drafts/issues/310#issuecomment-396765893">under discussion</a>.
 
 <h4 id="initial-letter-ancestors">