git-worktree.txt: consistently use term "working tree"

git-worktree.txt: consistently use term "working tree"

As originally composed, git-worktree.txt employed a mix of "worktree"
and "working tree" which was inconsistent and potentially confusing to
readers. bc483285b7 (Documentation/git-worktree: consistently use term
"linked working tree", 2015-07-20) undertook the task of employing the
term "working tree" consistently throughout the document and avoiding
"worktree" altogether for descriptive text. Since that time, some
instances of "worktree" have crept back in. Continue the work of
bc483285b7 by transforming these to "working tree", as well.

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