Fix documentation of default generated value behavior

[jorrit]

When reading about the default behavior of @GeneratedValue I noticed a mismatch in the basic-mapping docs. The introduction tells that PostgreSQL uses SERIAL by default, while later on it says sequence (which is the right one). This small PR fixes this.

[SenseException]

Yes, with the example above in the docs it should result in a sequence in PostgreSQL. Thank you for your fix.