range-diff: update stale summary of --no-dual-color

275267937b (range-diff: make dual-color the default mode, 2018-08-13)
replaced --dual-color with --no-dual-color but left the option's
summary untouched.  Rewrite the summary to describe --no-dual-color
rather than dual-color.

Helped-by: Jonathan Nieder <jrnieder@gmail.com>
Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Kyle Meyer <kyle@kyleam.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/range-diff.c

@@ -25,7 +25,7 @@ int cmd_range_diff(int argc, const char **argv, const char *prefix)
 		OPT_INTEGER(0, "creation-factor", &creation_factor,
 			    N_("Percentage by which creation is weighted")),
 		OPT_BOOL(0, "no-dual-color", &simple_color,
-			    N_("color both diff and diff-between-diffs")),
+			    N_("use simple diff colors")),
 		OPT_END()
 	};
 	int i, j, res = 0;