Update cheatsheets

ppmhist

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# ppmhist
+
+> Print a histogram of the colors present in a PPM image.
+> See also: `pgmhist`.
+> More information: <https://netpbm.sourceforge.net/doc/ppmhist.html>.
+
+- Generate the histogram for human reading:
+
+`ppmhist -nomap {{path/to/image.ppm}}`
+
+- Generate a PPM file of the colormap for the image, with the color histogram as comments:
+
+`ppmhist -map {{path/to/image.ppm}}`
+
+- Display version:
+
+`ppmhist -version`