doc: Fix broken link in doc/build-osx.md
doc: Fix broken link in doc/build-osx.md

This fixes the regression in PR #15964 as noted here:

https://github.com/bitcoin/bitcoin/pull/15964/files#r298798933

Former-commit-id: cd166ecdc11c3f8253ac3289d09d256bc39b2118
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin2,syscoin/syscoin2,syscoin/syscoin