[css-shapes-1][editorial] Add <shape()> to <basic-shape> (#13894)

Oversight in #9727.

Author: cdoublev

css-shapes-1/Overview.bs

@@ -331,7 +331,7 @@ Basic Shapes</h2>
 	can be specified using basic shape functions.
 
 	<pre class=prod>
-		<<basic-shape>> = <<basic-shape-rect>> | <<circle()>> | <<ellipse()>> |  <<polygon()>> | <<path()>>
+		<<basic-shape>> = <<basic-shape-rect>> | <<circle()>> | <<ellipse()>> |  <<polygon()>> | <<path()>> | <<shape()>>
 	</pre>
 
 	When using this syntax