Update cheatsheets

ppmshift

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# ppmshift
+
+> Shift the lines in a PPM image by a randomized amount.
+> More information: <https://netpbm.sourceforge.net/doc/ppmshift.html>.
+
+- Shift the lines in the input image by a randomized amount not exceeding s to the left or to the right:
+
+`ppmshift {{s}} {{path/to/input_file.ppm}} > {{path/to/output_file.ppm}}`