Minor issue in docs

[HenrikJStromberg]

Chapter 1.5 of the docs contains the following listing:

assert_eq!( root.data(), &9 );

let root = tree.root_mut();
*root.data_mut() = 0;

However it is missing mutability for root in order to compile:

assert_eq!( root.data(), &9 );

let mut root = tree.root_mut();
*root.data_mut() = 0;