Fix typo related to ZMQ in build docs

Summary: Fixes T873

Test Plan:

cmake -DBUILD_BITCOIN_ZMQ=OFF ..
grep BUILD_BITCOIN_ZMQ CMakeCache.txt   # verify it's actually off

Reviewers: #bitcoin_abc, nakihito, Fabien

Reviewed By: #bitcoin_abc, nakihito, Fabien

Maniphest Tasks: T873

Differential Revision: https://reviews.bitcoinabc.org/D6099