Use "sh" code block language specification

In order to indicate that a block contains shell code one should use
e.g. "sh". Under
https://github.com/github/linguist/blob/master/lib/linguist/languages.yml#L5602
the exhaustive list can be found.

The one file that already used "shell" was corrected. The indicator
"shell" is used for interactive session snippets. See
https://stackoverflow.com/a/49004070/8831116 for a reference.