git-worktree.txt: employ fixed-width typeface consistently

git-worktree.txt: employ fixed-width typeface consistently

git-worktree documentation generally does a good job of formatting
literal text using a fixed-width typeface, however, some instances of
unformatted literal text have crept in over time. Fix these.

While at it, also fix a few incorrect typefaces resulting from wrong
choice of Asciidoc quotes.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>