Do not generate RPC doc index files multiple times

Summary: See title. The loop is not constructed properly.

Test Plan:

build-configurations.py build-docs
cat abc-ci-builds/build-docs/doc/rpc/en/0.21.13/index.md
cat abc-ci-builds/build-docs/doc/rpc/en/0.21.13/rpc/index.md

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7071