Update README.md with corrected stub_node example

Updated README.me with working stub_node example.

Calling stub_node without the name option results in an exception being thrown 
```
Chef::Exceptions::ValidationFailed:
       Option name must be a kind of String! 
```

Attempting to directly update a node object without using .set, .normal, etc leads to the error

```
Chef::Exceptions::ImmutableAttributeModification:
       Node attributes are read-only when you do not specify which precedence level to set. To set an attribute use code like `node.default["key"] = "value"'
```