Fix spelling of "unreachable" in OP_CHECKMULTISIG

Summary: Wrong spelling of "unreachable" in the implementation of OP_CHECKMULTISIG/OP_CHECKMULTISIGVERIFY.

Test Plan: ninja check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, PiRK, Mengerian

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, PiRK, Mengerian

Subscribers: Mengerian, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D10126