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.  The rest of the documentation uses backticks
whenever a caret is encountered in running text, so do the same here.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>