Consider renaming --parent to --parents

### Is your feature request related to a problem? Please describe

Just upgraded to 0.16.
I noticed the --parent option with asdf set.

Now, the documentation/help text is actually clear, but from just the option, I thought it would set in the tool-version of the direct parent (which doesn't really make sense)

### Describe the proposed solution

Change --parent in --parents

### Describe similar `asdf` features and why they are not sufficient

asdf --parent

### Describe other workarounds you've considered

not applicable

Start at the `asdf set` command and search the codebase for `--parent` in the option definition, help text, and any related tests. Rename the option to `--parents`, update matching user-facing text, and verify that `asdf set --help` and the relevant command tests use the new spelling.

Written by the indexing model from the issue text.