Noted that mainEntity is subPropertyOf the about property.

Fixes #1681

Author: danbri

data/schema.rdfa

@@ -5669,6 +5669,7 @@ The place is __open__ if the [[opens]] property is specified, and __closed__ oth
       <span property="rdfs:comment">Indicates the primary entity described in some page or other CreativeWork.</span>
       <span>Domain: <a property="http://schema.org/domainIncludes" href="http://schema.org/CreativeWork">CreativeWork</a></span>
       <span>Range: <a property="http://schema.org/rangeIncludes" href="http://schema.org/Thing">Thing</a></span>
+      <link property="rdfs:subPropertyOf" href="http://schema.org/about" />
       <link property="http://schema.org/inverseOf" href="http://schema.org/mainEntityOfPage"/>
     </div>