Update 05.mutating-state.md

[bhgsbatista]

Replaced reference of this.state inside setState with function that takes prevState as parameter because this pattern avoids some nasty issues like:

this.setState({
  items: this.state.items.concat('lorem')
});

// items will not have 'lorem' here, oops
this.setState({
  items: this.state.items.concat('ipsum')
});