Update cheatsheets

sum

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# sum
+
+> Compute checksums and the number of blocks for a file.
+> A predecessor to the more modern `cksum`.
+> More information: <https://www.gnu.org/software/coreutils/sum>.
+
+- Compute a checksum with BSD-compatible algorithm and 1024-byte blocks:
+
+`sum {{path/to/file}}`
+
+- Compute a checksum with System V-compatible algorithm and 512-byte blocks:
+
+`sum --sysv {{path/to/file}}`