typo in unittest docs

[sethladd]

From a keen eyed developer:

See http://api.dartlang.org/unittest.html

I think:

import('path-to-dart/lib/unittest/unitest.dart');

is meant to be:

import('path-to-dart/lib/unittest/unittest.dart');

added a 't' in unittest.

[sethladd]

cc @sigmundch.

[gramster]

Added Fixed label.