Formula-Cookbook: Stop recommending rm_f in postinstall

- After all the work that went into
  #17705, we
  don't want the docs disagreeing with what CI says
  the style should be.

docs/Formula-Cookbook.md

@@ -995,7 +995,7 @@ class Foo < Formula
  url "https://example.com/foo-1.0.tar.gz"
 
  def post_install
- rm_f pkgetc/"cert.pem"
+ rm pkgetc/"cert.pem" if File.exist?(pkgetc/"cert.pem")
  pkgetc.install_symlink Formula["ca-certificates"].pkgetc/"cert.pem"
  end
  # ...