Change `grunt production` message

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Change grunt productionso that it instructs the developer to assign the spreadsheet issue to @brent-phet.

I.e., production.js, line 224:

- 2. Create an issue in the phet-io repo and assign to @kathy-phet to update the main "PhET-iO Simulations" spreadsheet from this new publication. (https://docs.google.com/spreadsheets/d/18_QNGuVtYtxOEKG9xRBs_PSQpyvzySF1Gk5puR-5Fv4/edit#gid=1881767354)
+ 2. Create an issue in the phet-io repo and assign to @brent-phet to update the main "PhET-iO Simulations" spreadsheet from this new publication. (https://docs.google.com/spreadsheets/d/18_QNGuVtYtxOEKG9xRBs_PSQpyvzySF1Gk5puR-5Fv4/edit#gid=1881767354)

Slack#phet-io discussion

Amy Rouinfar We’ve published 3 sims with PhET-iO this week (Geometric Optics, Geometric Optics: Basics, Density), but I don’t see them in the [PhET-iO Simulations spreadsheet](https://docs.google.com/spreadsheets/d/18_QNGuVtYtxOEKG9xRBs_PSQpyvzySF1Gk5puR-5Fv4/edit#gid=1881767354) that clients use to access the latest links. What’s the process supposed to be for adding sims? Brent Marcus I can do it, but I need Edit access to that file. Chris Malley The process is to create a GitHub issue to add it to the spreadsheet. `grunt production` instructs you to do that when it completes the deploy. Geometric Optics: https://github.com/phetsims/phet-io/issues/1945 Density: ? Kathy Perkins Hey Brent, If you are still around, let's do this now together, and then you can take this over ... Brent Marcus Ok here kathy Kathy Perkins I'll send a link directly Also sent to the channel Chris Malley Shall we modify the `grunt production` reminder to assign future issues to @brent-phet? Brent Marcus Yes, please pixelzoom

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@zepumph please review. Close if OK.