Migration Guide: Hibernate and JPA changes point to wrong ORM doc version

From gbadner@redhat.com:

Sande, I see that the link to Hibernate ORM 5.0 Migration Guide points to the master branch, which is currently for 5.1. It should instead point to the migration guide in 5.0 branch: https://github.com/hibernate/hibernate-orm/blob/5.0/migration-guide.adoc .

FYI, I plan to make updates to the migration guide for 5.0.9: https://hibernate.atlassian.net/browse/HHH-10542 .