[css-images-3] Fix typos in production rules (w3c#8776)

Author: cdoublev

css-images-3/Overview.bs

@@ -506,10 +506,10 @@ Radial Gradients: the ''radial-gradient()'' notation {#radial-gradients}
 	The radial gradient syntax is:
 
 	<pre class=prod>
-		<<radial-gradient()>> = radial-gradient( [ <radial-gradient-syntax> ] )
+		<<radial-gradient()>> = radial-gradient( [ <<radial-gradient-syntax>> ] )
 
 		<dfn>&lt;radial-gradient-syntax></dfn> =
-			<<rg-ending-shape>> || <<rg-size>> ]? [ at <<position>> ]? ,
+			[ <<rg-ending-shape>> || <<rg-size>> ]? [ at <<position>> ]? ,
 			<<color-stop-list>>
 
 		<dfn>&lt;rg-size></dfn> = <<rg-extent-keyword>> | <<length [0,∞]>> | <<length-percentage [0,∞]>>{2}