set errata-ai/vale separator

[hone]

The current vale action breaks when there is multiple files detected in the diff. You'll get the following warning:

Warning: User-specified path (content/community.md content/docs/_index.md) is invalid; falling back to 'all'.

It then passes the files with a space separator, but the vale action doesn't know how to handle it unless you specify the separator. I've hardcoded a separator , for both the changed-files and the vale-action actions so they match.

This should fix the vale error in #882.