Documentation: do not misinterpret pull refspec as bold text

Documentation: do not misinterpret pull refspec as bold text

Use the {asterisk} entity to avoid mistreating the asterisks
in "(e.g., refs/heads/*:refs/remotes/origin/*)" as delimiters
for bold text.

From a quick search with 'git grep -e "\*.*\*"', this seems to
be the last example of this particular formatting problem.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>