Update postgres plan in Heroku deployment docs

[theptrk]

Description

Changes the command to create a heroku postgres addon

$ heroku addons:create heroku-postgresql:mini

 ›   Warning: heroku update available from 8.11.5 to 9.2.1.
Creating heroku-postgresql:mini on ⬢ location-tracker... !
 ▸    We tried to create heroku-postgresql:mini, but received an error from the add-on provider. Try the request again, or log a support ticket and include
 ▸    this message: The mini plan has reached end-of-life. You can try again with a valid plan. See the available plans with: heroku addons:plans
 ▸    heroku-postgresql.

The next cheapest heroku option is "essentials-0"

documentation: https://elements.heroku.com/addons/heroku-postgresql

$ heroku addons:create heroku-postgresql:essential-0

 ›   Warning: heroku update available from 8.11.5 to 9.2.1.
Creating heroku-postgresql:essential-0 on ⬢ location-tracker... ~$0.007/hour (max $5/month)
Database should be available soon
postgresql-silhouetted-14783 is being created in the background. The app will restart when complete...
Use heroku addons:info postgresql-silhouetted-14783 to check creation progress
Use heroku addons:docs heroku-postgresql to view documentation

Checklist:

• I've made sure that tests are updated accordingly (especially if adding or updating a template option)
• I've updated the documentation or confirm that my change doesn't require any updates

Rationale

This option (mini) is now longer valid

[browniebroke]

Thanks! Notes that a while back but completely forgot it was in our docs.