Grammar improvement when more than one model per file

[AndrewLane]

Description

Grammar improvement to error message when more than model is defined in a file

Checklist

• [ x ] I have read the contributing guide and understand what's expected of me
• [ x ] I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• [ x ] This PR includes tests, or tests are not required/relevant for this PR
• [ x ] I have opened an issue to add/update docs, or docs changes are not required/relevant for this PR
• I have run changie new to create a changelog entry

[cla-bot]

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[AndrewLane]

Thanks for your pull request, and welcome to our community! We require contributors to sign our Contributor License Agreement and we don't seem to have your signature on file. Check out this article for more information on why we have a CLA.

In order for us to review and merge your code, please submit the Individual Contributor License Agreement form attached above above. If you have questions about the CLA, or if you believe you've received this message in error, don't hesitate to ping @drewbanin.

CLA has not been signed by users: @AndrewLane

Just signed it

[AndrewLane]

Re-opening to see if the bot will refresh the status

[jtcohen6]

@AndrewLane Sorry we missed this! It looks like the same improvement was flagged over in #6245 / #6251. I'm going to close this PR in favor of that one, but you'll get credit in my book for being the first to propose the fix.