Corrected the typo on comments.

The comments for `beforeEach` and `afterEach`  methods are not commented properly. It is commented as  both `beforeEach` and `afterEach` methods are executed every time Mocha is run, but AFAIK both `beforeEach` and `afterEach` methods will run on each individual tests. It will be `before` and `after` methods are executed every time Mocha is run.