Fix grammar errors

Hello!

`an Hash` is not correct.

You put `an` before vowels that is correct and also before words that starts with `h` with some exceptions.

If the `h` is unsounded use `an` like:

`an honorable library`
`an hour`

In both, they `h` doesn't sound but:

`a Hash`
`a Home`
`a Horse`

Those `h` does sound so you put an `a`. There are more rules but meh, this is Ruby, not English grammar.

Anyway, keep the hard work, lovely framework.