Update cheatsheets

tldr/diffimg

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# diffimg
+
+> Generate an image where each pixel is the difference between corresponding pixels in the 2 source images.
+> More information: <https://graphviz.org/pdf/diffimg.1.pdf>.
+
+- Compare two images and output the difference image to the specified file:
+
+`diffimg {{path/to/image1}} {{path/to/image2}} {{path/to/output_image}}`