[w3c/manifest] chore: update PULL_REQUEST_TEMPLATE.md (#916)

Update the template to reflect how we more commonly use it... and also to remind us to update the JSON schema. 
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/916

-- Commit Summary --

  * chore: update PULL_REQUEST_TEMPLATE.md

-- File Changes --

    M PULL_REQUEST_TEMPLATE.md (16)

-- Patch Links --

https://github.com/w3c/manifest/pull/916.patch
https://github.com/w3c/manifest/pull/916.diff

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