git-rerere.txt: Clarify ambiguity of the config variable

Use the less ambiguous
"set variable foo in order to enable bar"
rather than
"set variable foo to enable bar" which may trick users into
assuming that "enable" is a good value for "foo".

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rerere.txt

@@ -23,7 +23,7 @@ on the initial manual merge, and applying previously recorded
 hand resolutions to their corresponding automerge results.
 
 [NOTE]
-You need to set the configuration variable rerere.enabled to
+You need to set the configuration variable rerere.enabled in order to
 enable this command.