[RPC docs] Fix build paths

Summary: This patch fixes the build so that files are correctly named and located in their group directories.

Test Plan:
ninja doc-rpc
Before this patch, ls <build>/doc/rpc/en/0.21.10/rpc/ contains all of the (mis-named) files.
Post patch, they are named and located correctly.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6595