Remove last comma from schema.html template

[0urobor0s]

With the introduction of delimit in #683 it is possible to use it
instead of range while also removing the last comma in keywords tag.

[CLAassistant]

All committers have signed the CLA.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. The resources of the Hugo team are limited, and so we are asking for your help.
If this is a bug and you can still reproduce this error on the master branch, please reply with all of the information you have about it in order to keep the issue open.
If this is a feature request, and you feel that it is still relevant and valuable, please tell us why.
This issue will automatically be closed in the near future if no further activity occurs. Thank you for all your contributions.

[0urobor0s]

This still happens in master but is not critical.
The relevant url is https://schema.org/keywords

[github-actions]

This PR has been automatically marked as stale because it has not had recent activity. The resources of the Hugo team are limited, and so we are asking for your help.
Please check https://github.com/gohugoio/hugo/blob/master/CONTRIBUTING.md#code-contribution and verify that this code contribution fits with the description. If yes, tell is in a comment.
This PR will automatically be closed in the near future if no further activity occurs. Thank you for all your contributions.