worktree: use 'worktree' over 'working tree'

worktree: use 'worktree' over 'working tree'

It is helpful to distinguish between a 'working tree' and a 'worktree'.
A worktree contains a working tree plus additional metadata. This
metadata includes per-worktree refs and worktree-specific config.

This is the second of multiple changes to git-worktree.txt, restricted
to the COMMANDS section.

There is some language around the movement of "the working tree of a
linked worktree" which is used once, but the remaining uses are left as
just moving "a linked worktree" for brevity.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>