Update cheatsheets

pbmtogem

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# pbmtogem
+
+> Read a PBM image as input and produce a compressed GEM .img file as output.
+> `pbmtogem` cannot compress repeated lines.
+> More information: <https://netpbm.sourceforge.net/doc/pbmtogem.html>.
+
+- Convert a PBM image into a GEM .img file:
+
+`pbmtogem {{path/to/file.pbm}} > {{path/to/file.img}}`
+
+- Suppress all informational messages:
+
+`pbmtogem -quiet`
+
+- Display version:
+
+`pbmtogem -version`