Update cheatsheets

tldr/linux/systemctl-poweroff

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+---
+syntax: markdown
+tags: [tldr, linux]
+source: https://github.com/tldr-pages/tldr.git
+---
+# systemctl-poweroff
+
+> Power off the system.
+> See also: `poweroff`.
+> More information: <https://www.freedesktop.org/software/systemd/man/systemctl.html#poweroff>.
+
+- Power off the system:
+
+`systemctl poweroff`
+
+- Power off the system immediately without asking services to stop gracefully:
+
+`systemctl poweroff --force`
+
+- Power off the system immediately without sending notifications to logged-in users:
+
+`systemctl poweroff --force --no-wall`