Update cheatsheets

tldr/linux/let

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+---
+syntax: markdown
+tags: [tldr, linux]
+source: https://github.com/tldr-pages/tldr.git
+---
+# let
+
+> Evaluate arithmetic expressions in shell.
+> Supports variables, operators, and conditional expressions.
+> More information: <https://manned.org/let>.
+
+- Evaluate a simple arithmetic expression:
+
+`let "{{result = a + b}}"`
+
+- Use post-increment and assignment in an expression:
+
+`let "{{x++}}"`
+
+- Use conditional operator in an expression:
+
+`let "{{result = (x > 10) ? x : 0}}"`
+
+- Display help:
+
+`let --help`