git-cvsimport.txt: fix '-M' description.

Fix '-M' description. Old one reads as if the user can somehow "see"
the default regex when using -M along with -m.

Signed-off-by: Sergei Organov <osv@javad.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-cvsimport.txt

@@ -107,8 +107,8 @@ If you need to pass multiple options, separate them with a comma.
 
 -M <regex>::
 	Attempt to detect merges based on the commit message with a custom
-	regex. It can be used with '-m' to also see the default regexes.
-	You must escape forward slashes.
+	regex. It can be used with '-m' to enable the default regexes
+	as well. You must escape forward slashes.
 
 -S <regex>::
 	Skip paths matching the regex.