Make Style/AutoResourceCleanup examples equivalent

In the bad example, `f` is the `File` object, so in the good example it should be the same.
(Prior to this commit, `f` in the good example is the return value of the block passed to `File.open`.)

Author: duckinator

lib/rubocop/cop/style/auto_resource_cleanup.rb

@@ -13,7 +13,7 @@ module Style
       #   f = File.open('file')
       #
       #   # good
-      #   f = File.open('file') do
+      #   File.open('file') do |f|
       #     ...
       #   end
       class AutoResourceCleanup < Cop

manual/cops_style.md

@@ -147,7 +147,7 @@ resource cleanup.
 f = File.open('file')
 
 # good
-f = File.open('file') do
+File.open('file') do |f|
   ...
 end
 ```