Update bitcoindevkit.org links from `bdk` to `bdk_wallet`

[notmandatory]

          It will be a new link so I did need to update the README.

📌 After publishing I'll do a PR to update bitcoindevkit.org links too.

Originally posted by @notmandatory in bitcoindevkit/bdk#1326 (comment)

[notmandatory]

fixed by #176