docs: use short hand install syntax

Removes the need to do `tap` + `install`. Minor annoyance to me every time I install this on a new computer.

Love the project! ❤️

Author: jaredallard

README.md

@@ -29,8 +29,7 @@ launch agent so it starts at login.
 After installing [Homebrew](https://brew.sh), run:
 
 ```
-brew tap nextfire/tap
-brew install apple-music-discord-rpc
+brew install nextfire/tap/apple-music-discord-rpc
 brew services restart apple-music-discord-rpc
 ```