add missing '

Doc/lib/liblogging.tex

@@ -842,7 +842,7 @@ Currently, the useful mapping keys in a LogRecord are:
 Returns a new instance of the \class{Formatter} class. The
 instance is initialized with a format string for the message as a whole,
 as well as a format string for the date/time portion of a message. If
-no \var{fmt} is specified, \code{\%(message)s'} is used. If no \var{datefmt}
+no \var{fmt} is specified, \code{'\%(message)s'} is used. If no \var{datefmt}
 is specified, the ISO8601 date format is used.
 \end{classdesc}