Separate the mac instructions into two code blocks

This allows easily copy-pasting the download command without always copying the `brew install` commands.

README.md

@@ -47,10 +47,11 @@ curl -o- https://raw.githubusercontent.com/shawwn/llama-dl/56f50b96072f42fb2520b
 ```
 
 Mac:
-
 ```sh
 brew install bash
 brew install wget
+```
+```sh
 curl -o- https://raw.githubusercontent.com/shawwn/llama-dl/56f50b96072f42fb2520b1ad5a1d6ef30351f23c/llama.sh | $(brew --prefix)/bin/bash
 ```