Fix README using File.exist? instead of File.exists?

The latter is deprecated since Ruby 2.1.0.
ref: http://docs.ruby-lang.org/en/2.2.0/File.html#method-c-exists-3F

And `File.exist?` works fine for older Ruby versions.

Author: tricknotes

README.markdown

@@ -155,7 +155,7 @@ We can replace the mod_rewrite rules with the following Rack::Rewrite rule:
 ```ruby
 maintenance_file = File.join(RAILS_ROOT, 'public', 'system', 'maintenance.html')
 send_file /.*/, maintenance_file, :if => Proc.new { |rack_env|
-  File.exists?(maintenance_file) && rack_env['PATH_INFO'] !~ /\.(css|jpg|png)/
+  File.exist?(maintenance_file) && rack_env['PATH_INFO'] !~ /\.(css|jpg|png)/
 }
 ```