Update code examples in PK migration docs

What does this MR do?

We have the process of migrating PK columns to bigint documented in https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#migrating-integer-primary-keys-to-bigint.

These docs were created a while ago, and before big changes like DB decomposition and making batched background migrations globally available.

While working on #389306 (closed) I found that the code examples we have in the docs are outdated and result in an error. This MR updates them so they work as expected.