docs: add pkg-config to macOS prerequisites

Signed-off-by: Artem Lytvyn <alytvyn@redhat.com>

Author: letv1nnn

CONTRIBUTING.md

@@ -150,11 +150,11 @@ xcode-select --install
 
 ### Z3 installation
 
-The `openshell-prover` crate links against the system Z3 library via pkg-config.
+The `openshell-prover` crate links against the system Z3 library via `pkg-config`.
 
 ```bash
 # macOS
-brew install z3
+brew install z3 pkg-config
 
 # Ubuntu / Debian
 sudo apt install libz3-dev