Migration filename update in Readme

[zylinski]

The migration readme retained the old location and name of the migration file current.sql, which had been moved and renamed to 1-current.sql. This has been corrected.

Description

Performance impact

Security impact

Checklist

• [x] My code matches the project's code style and yarn lint:fix passes.
• [x] I've added tests for the new feature, and yarn test passes.
• [x] I have detailed the new feature in the relevant documentation.
• [x] I have added this feature to 'Pending' in the RELEASE_NOTES.md file (if one exists).
• [x] If this is a breaking change I've explained why.

[benjie]

Thanks for raising this; I've fixed and improved the documentation around this here: https://github.com/graphile/starter/pull/268