Update cheatsheets

pgmkernel

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# pgmkernel
+
+> Generate a convolution kernel to be used with `pnmconvol`.
+> See also: `pnmconvol`.
+> More information: <https://netpbm.sourceforge.net/doc/pgmkernel.html>.
+
+- Generate a convolution kernel:
+
+`pgmkernel {{width}} {{height}} > {{path/to/output.pgm}}`
+
+- Generate a quadratic convolution kernel:
+
+`pgmkernel {{size}} > {{path/to/output.pgm}}`
+
+- Specify the weight of the center in the generated kernel:
+
+`pgmkernel -weight {{value}} {{width}} {{height}} > {{path/to/output.pgm}}`