SF bug #764616: execfile(filename,...) not execfile(file,...)

Clarify parameter name.

Doc/lib/libfuncs.tex

@@ -354,7 +354,7 @@ class C:
   \function{execfile()}.
 \end{funcdesc}
 
-\begin{funcdesc}{execfile}{file\optional{, globals\optional{, locals}}}
+\begin{funcdesc}{execfile}{filename\optional{, globals\optional{, locals}}}
   This function is similar to the
   \keyword{exec} statement, but parses a file instead of a string.  It
   is different from the \keyword{import} statement in that it does not