Added missing "from" in IOError description.

Doc/lib/libexcs.tex

@@ -161,7 +161,7 @@ Raised when an \keyword{assert} statement fails.
   object) fails for an I/O-related reason, e.g., ``file not found'' or
   ``disk full''.
 
-  This class is derived \exception{EnvironmentError}.  See the
+  This class is derived from \exception{EnvironmentError}.  See the
   discussion above for more information on exception instance
   attributes.
 \end{excdesc}