Removed the redundant usage of layer.

Since DBAL is database abstraction layer, and is defined in the document as such, to me it reads funny to have it as database abstraction layer layer. It sounds nitpicky, I know, but I find it an easier read this way. Also, the possessive DBAL stuck out ("DBAL's layer") as the DBAL doesn't own a layer, it is the layer whereas Doctrine does own the layer and is properly defined in the header.
@WouterJ I can make the change, but I'm curious if you can explain your line of thinking a bit more so I can understand what you are looking for.

The reason I'm leaving Doctrine as possessive of the DBAL is that in the document, it is explained that the ORM component of Doctrine is a higher level of the DBAL. So Doctrine owns multiple components, making it possessive of the DBAL. That's my reasoning at least.