Avoid mutating state in the example code

According to react documentation it is advised to: NEVER mutate this.state directly, as calling setState() afterwards may replace the mutation you made. Treat this.state as if it were immutable. https://facebook.github.io/react/docs/animation.html In this particular case it is probably doesn't matter since setState is called directly after mutation, but it does provide a bad example of state mutation. Another way of removing an item from an array can be `newItems = this.state.slice(0,i).concat(this.state.slice(i+1))` however the meaning can be less obvious to some.