Minor editorial rephrasing to clarify

Will prevent future misunderstandings like #567

Author: TallTed

index.html

@@ -1404,8 +1404,8 @@ <h3>Algorithm</h3>
                   <span class="changed">the result of
                     <a>IRI expanding</a> <var>value</var>
                     using <code>true</code> for <var>document relative</var>
-                  </span>.
-                  If it is not an <a>IRI</a>, or a <a>blank node identifier</a>, an
+                  </span>;
+                  if <var>value</var> is neither an <a>IRI</a> nor a <a>blank node identifier</a>, an
                   <a data-link-for="JsonLdErrorCode">invalid vocab mapping</a>
                   error has been detected and processing is aborted.
                   <div class="note">The use of <a>blank node identifiers</a> to value for <code>@vocab</code> is obsolete,