docs/Formula-Cookbook: Fix outdated HEAD hash syntax

- Using the previous syntax, `brew audit --strict` fails because this
  uses the old Ruby hash syntax.

docs/Formula-Cookbook.md

@@ -577,10 +577,9 @@ To use a specific commit, tag, or branch from a repository, specify [`head`](htt
 
 ```ruby
 class Foo < Formula
-  head "https://github.com/some/package.git", :revision => "090930930295adslfknsdfsdaffnasd13"
-                                         # or :branch => "develop" (the default is "master")
-                                         # or :tag => "1_0_release",
-                                         #    :revision => "090930930295adslfknsdfsdaffnasd13"
+  head "https://github.com/some/package.git", revision: "090930930295adslfknsdfsdaffnasd13"
+                                         # or branch: "main" (the default is "master")
+                                         # or tag: "1_0_release", revision: "090930930295adslfknsdfsdaffnasd13"
 end
 ```