docs: migration docs mention v6.0.0 but latest is v5.2.x

[PJ Fanning]

See https://doc.akka.io/docs/akka-persistence-jdbc/current/migration.html - maybe the first section should be about v5.2.0 instead of v6.0.0.

[Johan Andrén]

Thanks for letting us know, seems like a mistake, it should indeed be 5.2.0