doc: correct route map match for prefix lists

doc: correct route map match for prefix lists

This corrects the route map documentation to add the missing "prefix-list"
keyword, which is necessary when matching against a prefix list (as opposed to
an access list).

Additionally, change hyphens for underscores in the variables the user is
supposed to substitute in those commands, to prevent any confusion with the
"prefix-list" keyword itself, and also to make it more consistent with the
other documented commands (which are already using underscores).

Signed-off-by: Tore Anderson <tore@redpill-linpro.com>