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 fifth of multiple changes to git-worktree.txt, restricted to
the CONFIGURATION FILE section.

While here, clear up some language to improve readability.

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