fetch-options.txt: prevent a wildcard refspec from getting misformatted

When explaining the "--tags" option as an equivalent to giving an explicit "refs/tags/*:refs/tags/*" refspec, the two asterisks were misinterpreted by AsciiDoc as a request to typeset the string segment between them in bold. We could fix it in two ways. We can replace them with {asterisk}s while keeping the string as body text, or we can mark it as a literal string with backquotes around it. Let's do the latter, as it is teaching the user an "exactly as typed" alternative. Noticed-by: Ramkumar Ramachandra <artagnon@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>