Fixup RepositoryProvider docs
Fixup RepositoryProvider docs

`Repository` is a private type, and rustdoc rightly complains when
public docs refer to them.  This change updates the reference to point
to `Client`, which is the pub type that effectively enforces the
constraints referenced in the docs.

heartsucker/rust-tuf,heartsucker/rust-tuf,heartsucker/rust-tuf