Update cheatsheets

tldr/linux/systemctl-set-default

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+---
+syntax: markdown
+tags: [tldr, linux]
+source: https://github.com/tldr-pages/tldr.git
+---
+# systemctl set-default
+
+> Symlink the `default.target` alias to the given target unit.
+> More information: <https://www.freedesktop.org/software/systemd/man/systemctl.html#set-default%20TARGET>.
+
+- Set `systemd`'s default boot mode:
+
+`systemctl set-default {{target_name.target}}`
+
+- Set `systemd` to boot to GUI mode by default:
+
+`systemctl set-default graphical.target`
+
+- Set `systemd` to boot to CLI mode by default:
+
+`systemctl set-default multi-user.target`