Proposer preparation data quoted validator index in API (#3080)
Proposer preparation data quoted validator index in API (#3080)

## Issue Addressed

#3077

## Proposed Changes

Quotes around validator index in `prepare_beacon_proposer` endpoint


Co-authored-by: realbigsean <d7e19930cc1f42c2d0781f4d9e6f1fe5891bf9cf@sigmaprime.io>

sigp/lighthouse,sigp/lighthouse,sigp/lighthouse,sigp/lighthouse