Add vertical line for breathing

docs/running-headscale-linux.md

@@ -55,6 +55,8 @@ from the [headscale repository](../)
 
 This command will start `headscale` in the current terminal session.
 
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 To continue the tutorial, open a new terminal and let it run in the background.
 Alternatively use terminal emulators like [tmux](https://github.com/tmux/tmux) or [screen](https://www.gnu.org/software/screen/).