Use correct syntax for brew installation

As per https://github.com/ericchiang/pup/issues/161#issuecomment-944005044

README.md

@@ -17,7 +17,7 @@ If you have Go installed on your computer just run `go get`.
 
 If you're on OS X, use [Homebrew](http://brew.sh/) to install (no Go required).
 
-    brew install https://raw.githubusercontent.com/EricChiang/pup/master/pup.rb
+    brew install pup
 
 ## Quick start