Update cheatsheets

pgmnoise

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# pgmnoise
+
+> Generate white noise.
+> More information: <https://netpbm.sourceforge.net/doc/pgmnoise.html>.
+
+- Generate a PGM image containing white noise:
+
+`pgmnoise {{width}} {{height}} > {{path/to/output.pgm}}`
+
+- Specify the seed for the pseudo-random number generator:
+
+`pgmnoise {{width}} {{height}} -randomseed {{value}} > {{path/to/output.pgm}}`