SF bug 799367: grammar error

Doc/lib/libdatetime.tex

@@ -819,7 +819,7 @@ Instance methods:
   \code{\var{dt}.replace(tzinfo=None)}.
 
   Note that the default \method{tzinfo.fromutc()} method can be overridden
-  in a \class{tzinfo} subclass to effect the result returned by
+  in a \class{tzinfo} subclass to affect the result returned by
   \method{astimezone()}.  Ignoring error cases, \method{astimezone()}
   acts like: