[doc] Minor corrections to osx dependencies

Summary:

Protobuf no longer has a --c++11 option, it's now the default.
qt5 is now just qt (5.8).

Backport of core PR10260
https://github.com/bitcoin/bitcoin/pull/10260/files

Test Plan:
Read it (it needs more backports to actually work, see
https://github.com/Bitcoin-ABC/bitcoin-abc/issues/338)

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3904