[backport#15559] doc: correct analysepsbt rpc doc

Summary:
rpc: return a number for estimated_feerate in analyzepsbt

Backport of Core PR15559

Test Plan:

ninja
test_runner.py rpc_psbt

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6650