fix: add missing macOS brew command

that's the equivalent of the MacPorts command to the "Using YubiKey" section.

Signed-off-by: David Xia <david@davidxia.com>

README.md

@@ -1033,6 +1033,12 @@ sudo systemctl enable --now pcscd.service
 
 **macOS**
 
+```console
+brew install gnupg
+```
+
+Or using MacPorts
+
 ```console
 sudo port install gnupg2 pcsc-tools
 ```