Update cheatsheets

tldr/usleep

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# usleep
+
+> Delay execution for a specific interval in microseconds.
+> Largely deprecated in favor of `nanosleep`.
+> See also: `sleep`, `nanosleep`.
+> More information: <https://manned.org/usleep.1>.
+
+- Delay in microseconds:
+
+`usleep {{microseconds}}`
+
+- Execute a specific command after a 500,000 microseconds delay:
+
+`usleep 500000 && {{command}}`