Adjust markup to worm around tool limitations; the "m" in "model" was being

dropped in the HTML formatted version.
Reported by Mike Coleman.

Doc/ref/ref4.tex

@@ -171,7 +171,7 @@ statement.  The \keyword{try} ... \keyword{finally} statement
 specifies cleanup code which does not handle the exception, but is
 executed whether an exception occurred or not in the preceding code.
 
-Python uses the ``termination'' \index{termination model}model of
+Python uses the ``termination''\index{termination model} model of
 error handling: an exception handler can find out what happened and
 continue execution at an outer level, but it cannot repair the cause
 of the error and retry the failing operation (except by re-entering