Remove unnecessary :require option from Gemfile example

`Bundler.require` automatically loads lib/rack/cors.rb even if there is no :require option.

This can be verified by the following code:

```rb
require "bundler/inline"

gemfile do
  source "https://rubygems.org"

  gem "rack-cors"
end

puts defined?(Rack::Cors)
```

```
$ ruby example.rb
constant
```

cf. https://guides.rubygems.org/name-your-gem/

README.md

@@ -13,7 +13,7 @@ Install the gem:
 Or in your Gemfile:
 
 ```ruby
-gem 'rack-cors', require: 'rack/cors'
+gem 'rack-cors'
 ```