Update deprecated functions in Box::leak example

[anselbobrow]

Fixes #1922

Previously, this example wasn't compiling because of the deprecated thread_rng and fill functions. Upon making the changes the compiler suggests and swapping usize for u64, the example compiles properly. Here are the compiler errors:

Compiling playground v0.0.1 (/playground)
warning: unused import: `rand::Fill`
 --> src/main.rs:2:5
  |
2 | use rand::Fill;
  |     ^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` (part of `#[warn(unused)]`) on by default

warning: use of deprecated function `rand::thread_rng`: Renamed to `rng`
 --> src/main.rs:5:25
  |
5 |     let mut rng = rand::thread_rng();
  |                         ^^^^^^^^^^
  |
  = note: `#[warn(deprecated)]` on by default

error[E0599]: no method named `try_fill` found for struct `Box<[{integer}; 100]>` in the current scope
 --> src/main.rs:7:11
  |
7 |     boxed.try_fill(&mut rng).unwrap();
  |           ^^^^^^^^
  |
help: there is a method `fill` with a similar name
  |
7 -     boxed.try_fill(&mut rng).unwrap();
7 +     boxed.fill(&mut rng).unwrap();
  |

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