Fix setexcessiveblock rpc help text

Summary:
Help text did not contain any information about its arguments despite
requiring one.

Test Plan:

ninja
./bitcoind
./bitcoin-cli help setexcessiveblock

Verify change in help text.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5567