Update cheatsheets

ppmflash

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# ppmflash
+
+> Brighten a PPM image file.
+> More information: <https://netpbm.sourceforge.net/doc/ppmflash.html>.
+
+- Generate a PPM image as output that is `flashfactor` times brighter than the input PPM image:
+
+`ppmflash {{flashfactor}} {{path/to/file.ppm}} > {{path/to/file.ppm}}`
+
+- Display version:
+
+`ppmflash -version`