Update cheatsheets

pgmdeshadow

@@ -0,0 +1,13 @@
+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# pgmdeshadow
+
+> Deshadow a PGM image.
+> More information: <https://netpbm.sourceforge.net/doc/pgmdeshadow.html>.
+
+- Remove grey shadows from a PGM image:
+
+`pgmdeshadow {{path/to/input_file.pgm}} > {{path/to/output_file.pgm}}`