Update term definition to allow IRIs and things that look like a keyword, other than '@context'.

Author: gkellogg

spec/latest/json-ld-syntax/index.html

@@ -560,7 +560,7 @@ <h3>The Context</h3>
   in an JSON document, to <tref>IRI</tref>s. A <tdef>term</tdef> is a short word that MAY be expanded to an
   <tref>IRI</tref>. A <tref>term</tref> MUST have the lexical form of <cite><a
   href="http://www.w3.org/TR/2009/REC-xml-names-20091208/#NT-NCName">NCName</a></cite> (see [[XML-NAMES]]),
-  <tref>compact IRI</tref>, <tref>absolute IRI</tref>, or
+  an '@' followed by an NCName (other than <code>@context</code>), <tref>compact IRI</tref>, <tref>absolute IRI</tref>, or
   be an empty string.</p>
 <p>The Web uses <tref title="IRI">IRIs</tref> for unambiguous identification. The
   idea is that these <tref>term</tref>s mean something that may be of use to other developers and that it is useful to