Address typo in ellipse() syntax

An ellipse may have an x-radius and y-radius, or neither.

resolves #3609

Author: ericwilligers

css-shapes-1/Overview.bs

@@ -376,7 +376,7 @@ Supported Shapes</h3>
 			</ul>
 		</dd>
 		<dt><dfn>ellipse()</dfn> =
-			ellipse( <<shape-radius>>{2}? [ at <<position>> ]? )
+			ellipse( [ <<shape-radius>>{2} ]? [ at <<position>> ]? )
 		</dt>
 		<dd>
 			<ul>