Fix the type alias for deadpool's BuildError

[fluxxu]

If I understand it correctly, diesel_async::pooled_connection::deadpool::BuildError is the error returned when
deadpool::managed::PoolBuilder::build failed.
According to the docs, the error should be BuildError<M::Error> where M is AsyncDieselConnectionManager, so the error should be
deadpool::managed::BuildError<<AsyncDieselConnectionManager as deadpool::managed::Manager>::Error> which is

deadpool::managed::BuildError<diesel_async::pooled_connection::PoolError>,

but in the current code, it is an alias of

deadpool::managed::BuildError<deadpool::managed::PoolError<diesel_async::pooled_connection::PoolError>>

[weiznich]

Thanks for opening this PR. I potentially plan to relicense diesel-async in the future. Because of that I need an explicitly written consent that you transfer the ownership of this change to me. Please add that to the commit message.

(Hit the wrong button…)

[fluxxu]

I consent that I transfer the ownership of this change to you.