Add apparently missing word for InvalidStateError definition.

Closes bug #973901.  Thanks Brian Gough.

Doc/lib/xmldom.tex

@@ -760,7 +760,7 @@ appropriate value for the \member{code} attribute.
 \end{excdesc}
 
 \begin{excdesc}{InvalidStateErr}
-  Raised when an attempt is made to use an object that is not or is no
+  Raised when an attempt is made to use an object that is not defined or is no
   longer usable.
 \end{excdesc}