doc: install qt5 when building on macOS

Brew has updated such that qt now refers to Qt 6.0.1. If builders
install this, configure will not work pick up qt. For now, install
qt@5 (5.15.2), until required build system and likely source changes
are made.

Author: fanquake

doc/build-osx.md

@@ -19,7 +19,7 @@ Then install [Homebrew](https://brew.sh).
 
 ## Dependencies
 ```shell
-brew install automake libtool boost miniupnpc libnatpmp pkg-config python qt libevent qrencode
+brew install automake libtool boost miniupnpc libnatpmp pkg-config python qt@5 libevent qrencode
 ```
 
 If you run into issues, check [Homebrew's troubleshooting page](https://docs.brew.sh/Troubleshooting).