doc: migrate from pkg-config to pkgconf in macOS build docs

Brew has migrated to using the later:
```bash
brew info pkg-config
==> pkgconf: stable 2.3.0 (bottled), HEAD
Package compiler and linker metadata toolkit
https://github.com/pkgconf/pkgconf
```

doc/build-osx.md

@@ -48,7 +48,7 @@ See [dependencies.md](dependencies.md) for a complete overview.
 To install, run the following from your terminal:
 
 ``` bash
-brew install cmake boost pkg-config libevent
+brew install cmake boost pkgconf libevent
 ```
 
 ### 4. Clone Bitcoin repository