Rename variable in tutorial

Renamed `owned_box` to `on_the_heap` to use a consistent
naming across the tutorial and the life time guide.
Also it makes the example easier to grasp.
auto merge of #14566 : cburgdorf/rust/patch-1, r=cmr

Renamed `owned_box` to `on_the_heap` to use a consistent
naming across the tutorial and the life time guide.
Also it makes the example easier to grasp.