[css-animations] Minor typo

Author: Sylvain Galineau

css-animations/Overview.bs

@@ -971,7 +971,7 @@ The <code>appendRule</code> method</h4>
 		<dt><dfn argument for="CSSKeyFramesRule/appendRule()">rule</dfn> of type <a interface>DOMString</a>
 		<dd>
 			The rule to be appended, expressed in the same syntax as one entry in the
-			''@keyframes'' rule. A valid rule is always appended e.g. even if its key already
+			''@keyframes'' rule. A valid rule is always appended e.g. even if its key(s) already
 			exists.
 	</dl>