Improve usage example for model generator decimal field

We should wrap parameters to quotes when pass parameters with curly braces to decimal generator because the shell (tested in `bash` and `zsh`) transforms them to several params. Example:

```shell
$ echo p{1,2}
p1 p2
```

So actual example (`rails generate model product price:decimal{10,2}`) generates migration like this:

```ruby
class CreateProducts < ActiveRecord::Migration
  def change
    create_table :products do |t|
      t.decimal10 :price
      t.decimal2 :price

      t.timestamps
    end
  end
end
```

Fixed version of the command (`rails generate model product 'price:decimal{10,2}'`) generates correct migration:

```ruby
class CreateProducts < ActiveRecord::Migration
  def change
    create_table :products do |t|
      t.decimal :price, precision: 10, scale: 2

      t.timestamps
    end
  end
end
```