Fix padrino-cache example comment

If memcache server is specified, it must be set options hash with `:server` key
ref: https://github.com/minad/moneta#creating-a-store

````
store = Moneta.new(:Memcached, server: 'localhost:11211')
````

### Right case

````
Padrino.cache = Padrino::Cache.new(:Memcached, :server => '127.0.0.1:11211', :exception_retry_limit => 1
````

### In such a case...

````
Padrino.cache = Padrino::Cache.new(:Memcached, '127.0.0.1:11211', :exception_retry_limit => 1
````

#### raise error.

````
  ERROR -  TypeError: no implicit conversion of Symbol into String; /path/to/vendor/bundle/ruby/2.1.0/gems/padrino-cache-0.12.0/lib/padrino-cache.rb:112:in `include?'
  ERROR -  TypeError: no implicit conversion of Symbol into String; /path/to/vendor/bundle/ruby/2.1.0/gems/padrino-cache-0.12.0/lib/padrino-cache.rb:112:in `include?'
/path/to/vendor/bundle/ruby/2.1.0/gems/padrino-cache-0.12.0/lib/padrino-cache.rb:112:in `include?': no implicit conversion of Symbol into String (TypeError)
	from /path/to/vendor/bundle/ruby/2.1.0/gems/padrino-cache-0.12.0/lib/padrino-cache.rb:112:in `new'
````