README.md: be explicit about kill-server
README.md: be explicit about kill-server

I would not want someone to think that `tmux kill-server` could be
used to simply "Restart [their] *tmux* server" (i.e. the topic of
the list item) without losing all of their existing sessions,
windows, panes, processes, etc.

Also, kill trailing whitespace introduced in the commit that
introduced `kill-server`.

lembacon/tmux-MacOSX-pasteboard,ChrisJohnsen/tmux-MacOSX-pasteboard,tjt263/tmux-MacOSX-pasteboard,bradparks/tmux-MacOSX-pasteboard