docs/credential-store: s/--store/--file/

The option name "--store" was used early in development, but
never even made it into an applied patch, let alone a
released version of git. I forgot to update the matching
documentation at the time, though.

Noticed-by: Jesse Hopkins <jesse.hopkins@lmco.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-credential-store.txt

@@ -29,7 +29,7 @@ linkgit:gitcredentials[7] or `EXAMPLES` below.
 OPTIONS
 -------
 
---store=<path>::
+--file=<path>::
 
 	Use `<path>` to store credentials. The file will have its
 	filesystem permissions set to prevent other users on the system