Fix typo reported to python-docs.

Doc/ref/ref6.tex

@@ -533,7 +533,7 @@ first two objects are used to determine the \emph{type} and
 \emph{value} of the exception.
 
 If the first object is an instance, the type of the exception is the
-class of the instance, the instance itself if the value, and the
+class of the instance, the instance itself is the value, and the
 second object must be \code{None}.
 
 If the first object is a class, it becomes the type of the exception.