Update deprecated functions in Box::leak example

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 compile errors:

```sh
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();
  |

For more information about this error, try `rustc --explain E0599`.
warning: `playground` (bin "playground") generated 2 warnings
error: could not compile `playground` (bin "playground") due to 1 previous error; 2 warnings emitted
```

Author: anselbobrow

src/scope/lifetime/static_lifetime.md

@@ -73,16 +73,16 @@ live for the entire duration, but only from the leaking point onward.
 extern crate rand;
 use rand::Fill;
 
-fn random_vec() -> &'static [usize; 100] {
-    let mut rng = rand::thread_rng();
+fn random_vec() -> &'static [u64; 100] {
+    let mut rng = rand::rng();
     let mut boxed = Box::new([0; 100]);
-    boxed.try_fill(&mut rng).unwrap();
+    boxed.fill(&mut rng);
     Box::leak(boxed)
 }
 
 fn main() {
-    let first: &'static [usize; 100] = random_vec();
-    let second: &'static [usize; 100] = random_vec();
+    let first: &'static [u64; 100] = random_vec();
+    let second: &'static [u64; 100] = random_vec();
     assert_ne!(first, second)
 }
 ```