docs: fix two broken examples in transports.md

[Aiudadadadf]

Found two broken examples in docs/advanced/transports.md that would confuse anyone trying to run them.

The first is in the "Custom transports" section. It references httpx.Mounts() as a class, which doesn't exist - the correct way to use mounts is passing a dict to Client(mounts=...). The example also had a missing comma between dict entries, and then passed the result in as transport=transport instead of mounts=mounts. Fixed it to use Client(mounts={...}) inline so there's no ambiguity.

The second is in the "Mounting transports" section. The HTTPSRedirectTransport class had handle_request(self, method, url, headers, stream, extensions) returning a tuple (303, headers, stream, extensions) - that's the old low-level transport API from a previous major version. The current BaseTransport.handle_request takes a single Request object and returns a Response. Rewrote it to match, using request.url.copy_with(scheme="https") and httpx.Response(303, ...) the same way the nearby HTTPSRedirect example already does it correctly.