repair string literal.

Doc/lib/liboptparse.tex

@@ -518,7 +518,7 @@ program, i.e. \code{os.path.basename(sys.argv{[}0])}.  The expanded string
 is then printed before the detailed option help.
 
 If you don't supply a usage string, \module{optparse} uses a bland but sensible
-default: \code{usage: {\%}prog {[}options]"}, which is fine if your script
+default: \code{"usage: {\%}prog {[}options]"}, which is fine if your script
 doesn't take any positional arguments.
 
 \item {}