Resolve unused_parens compilation warning

Resolve unused_parens compilation warning

Before this commit, the first "A Rust library" code sample produced
the following compilation warning:
```
test.rs:7:22: 7:36 warning: unnecessary parentheses around `for` head
expression, #[warn(unused_parens)] on by default
test.rs:7             for _ in (0..5_000_000) {
```

This commit just removes the parens around the range 0..5_000_000.