Update cheatsheets

tldr/linux/systemctl-mask

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+---
+syntax: markdown
+tags: [tldr, linux]
+source: https://github.com/tldr-pages/tldr.git
+---
+# systemctl mask
+
+> Link units to `/dev/null` so that they can be even started.
+> More information: <https://www.freedesktop.org/software/systemd/man/latest/systemctl.html#mask%20UNIT%E2%80%A6>.
+
+- Mask a service:
+
+`systemctl mask {{service_name}}`
+
+- Ensure that the service is shut down while masking:
+
+`systemctl mask {{service_name}} --now`