Removed doubled wording.

The guide previously stated:

> The compiler will automatically convert a box box point to a reference like &amp;point.

This fixes the doubled word `box`, so the statement reads

> The compiler will automatically convert a box point to a reference like &amp;point.

The code it is referring to is `compute_distance(&on_the_stack, on_the_heap);`, so a single `box` is appropriate.
auto merge of #14805 : zzmp/rust/patch-1, r=alexcrichton

The guide previously stated:

> The compiler will automatically convert a box box point to a reference like &amp;point.

This fixes the doubled word `box`, so the statement reads

> The compiler will automatically convert a box point to a reference like &amp;point.

The code it is referring to is `compute_distance(&on_the_stack, on_the_heap);`, so a single `box` is appropriate.