[css-borders-4] Accept infinity keyword in superellipse() (w3c#11638)

This was already specified in prose but missing in the production rule.

Author: cdoublev

css-borders-4/Overview.bs

@@ -366,7 +366,7 @@ Issue <a href="https://github.com/w3c/csswg-drafts/issues/11610">#11610</a>: che
 'corner-shape' values</h4>
 
 	<pre class=prod>
-		<dfn><<corner-shape-value>></dfn> = ''round'' | ''scoop'' | ''bevel'' | ''notch'' | ''straight'' | ''squircle'' | superellipse(<<number [0,&infin;]>>)
+		<dfn><<corner-shape-value>></dfn> = ''round'' | ''scoop'' | ''bevel'' | ''notch'' | ''straight'' | ''squircle'' | superellipse(<<number [0,&infin;]>> | infinity)
 	</pre>
 
 	<dl dfn-type="value" dfn-for="<corner-shape-value>">