docs: fix missing DataSource in migrations config

[maciej-ka]

Motivation

Running migrations using current documentation ends with error: Error: Given data source file must contain export of a DataSource instance

Pull-Request Checklist

• [x] Code is up-to-date with the master branch
• [x] npm run format to apply prettier formatting
• npm run test passes with this change N/A
• This pull request links relevant issues as Fixes #0000 N/A
• There are new or updated unit tests validating the change N/A
• Documentation has been updated to reflect this change N/A
• The new commits follow conventions explained in CONTRIBUTING.md N/A

[coveralls]

coverage: 72.343% (-0.02%) from 72.359% when pulling 80fb8699af92fad3bfa57e8074a0c3c20d1cc556 on maciej-ka:docs-fix-migration-config into 9d1d3f1008e9c1f3488fc223a7853605df7f00dc on typeorm:master.