Basic usage documentation - minor error

[JeromeGill]

"using an age" example is slightly wrong,

// Get cache item.
    $item = $pool->getItem('path/to/item');

// Using an age.
$data = $pool->get();

should be

// Get cache item.
    $item = $pool->getItem('path/to/item');

// Using an age.
$item = $pool->get();

sorry I didn't see if you can PR the docs directly

[limenet]

@JeromeGill you can submit a PR against the docs in the separate repo for the docs