Update cheatsheets

pgmbentley

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# pgmbentley
+
+> Bentleyize a PGM image.
+> More information: <https://netpbm.sourceforge.net/doc/pgmbentley.html>.
+
+- Apply the Bentley Effect on a PGM image:
+
+`pgmbentley {{path/to/input_file.pgm}} > {{path/to/output_file.pgm}}`