Added quotes when defining the precision for the `decimal` fields

before:

```bash
$ rails generate migration AddDetailsToProducts price:decimal{5,2} supplier:references{polymorphic}
```

after:

```bash
$ rails generate migration AddDetailsToProducts 'price:decimal{5,2}' supplier:references{polymorphic}
```

guides/source/migrations.md

@@ -305,7 +305,7 @@ braces. You can use the following modifiers:
 For instance, running
 
 ```bash
-$ rails generate migration AddDetailsToProducts price:decimal{5,2} supplier:references{polymorphic}
+$ rails generate migration AddDetailsToProducts 'price:decimal{5,2}' supplier:references{polymorphic}
 ```
 
 will produce a migration that looks like this