rpc: add missing HelpExampleRpc for getblockfilter

Summary:

From all RPCs in the "blockchain" category, getblockfilter is the only one where there is only a CLI example present but not a curl RPC example (all other RPCs in this category have either both or none). This PR adds the missing HelpExampleRpc string.

This is a backport of Core PR18396

Test Plan: src/bitcoin-cli help getblockfilter

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8859