Update cheatsheets

ppmrainbow

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# ppmrainbow
+
+> Generate a rainbow.
+> More information: <https://netpbm.sourceforge.net/doc/ppmrainbow.html>.
+
+- Generate a rainbow consisting of the specified colors:
+
+`ppmrainbow {{color1 color2 ...}} > {{path/to/output_file.ppm}}`
+
+- Specify the size of the output in pixels:
+
+`ppmrainbow -width {{width}} -height {{height}} {{color1 color2 ...}} > {{path/to/output_file.ppm}}`
+
+- End the rainbow with the last color specified, do not repeat the first color:
+
+`ppmrainbow -norepeat {{color1 color2 ...}} > {{path/to/output_file.ppm}}`