Documentation: fix missing text for rev-parse --verify

The caret (^) is used as a markup symbol in AsciiDoc.  Due to the
inability of AsciiDoc to parse a line containing an unmatched caret, it
omitted the line from the output, resulting in the man page missing the
end of a sentence.  Escape this caret so that the man page ends up with
the complete text.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>