gpg(docs): use correct --verify syntax

The gpg --verify usage example within the 'gpg.program' variable
reference provides an incorrect example of the gpg --verify command
arguments.

The command argument order, when providing both a detached signature
and data, should be signature first and data second:
https://gnupg.org/documentation/manuals/gnupg/Operational-GPG-Commands.html

Signed-off-by: Robert Morgan <robert.thomas.morgan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: rtmorgan

Documentation/config/gpg.txt

@@ -2,7 +2,7 @@ gpg.program::
 	Use this custom program instead of "`gpg`" found on `$PATH` when
 	making or verifying a PGP signature. The program must support the
 	same command-line interface as GPG, namely, to verify a detached
-	signature, "`gpg --verify $file - <$signature`" is run, and the
+	signature, "`gpg --verify $signature - <$file`" is run, and the
 	program is expected to signal a good signature by exiting with
 	code 0, and to generate an ASCII-armored detached signature, the
 	standard input of "`gpg -bsau $key`" is fed with the contents to be