[css-content] fix propdef, see #216

Author: dauwhe

css-content/Overview.bs

@@ -80,7 +80,7 @@ figure:not([alt]) { content: attr(href, url), contents; }
 
 <pre class='propdef'>
     Name: content
-    Value: [ [ <<image>> | <<url>> ] ',' ]* [ normal | none | <<content-list>> ] [/ <<string>> ]?
+    Value: [ <<image>> ',' ]* [ normal | none | <<content-list>> ] [/ <<string>> ]?
     Initial: normal
     Applies To: ''::before'', ''::after'', ''::marker'',  and page margin boxes. Image and url values can apply to all elements. 
     Inherited: no