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.

Shoud have been part of commit 0b13ee0

Regarding rust-lang#29063: Replace 'iterator adapters' appearances to
'iterator adaptors', thus embracing the terminology used along the
API docs and achieving consistency between both sources.

r? @steveklabnik

None

Shoud have been part of commit 0b13ee0

…ning, r=alexcrichton

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 thereby removing the compilation warning.

…bnik

Regarding [rust-lang#29063 _[Docs] Terminology inconsistency between 'iterator adapters' and 'iterator adaptors'_](rust-lang#29063) :

This PR replaces 'iterator adapters' appearances (in TRPL book) to 'iterator adaptors', thus embracing the terminology used along the API docs and achieving consistency between both sources.

r? @alexcrichton