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

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>