Documentation: Fix mark-up of lines with more than one tilde

Documentation: Fix mark-up of lines with more than one tilde

The manual pages of cherry-pick and revert had examples with two revisions
on the same line in the examples section, that looked like this:

    git cherry-pick master~4 master~2::

Unfortunately, this is taken as a mark-up to make the part between two
tildes, "4 master", subscript.  Use {tilde} to make it explicit that we
do want ~ characters in these places (backslash does not help).

Reported-by: Sylvain Rabot <sylvain.rabot@f-secure.com>
Helped-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>