Fix typo: change 'successfull' to 'successful'

[munckymagik]

Hi @antonmi

Excellent project :+1:

I noticed a typo and this PR fixes that (see https://www.google.co.uk/search?q=define%20successfull)

Note:

• I wasn't able to run either mix test or mix espec without errors. In the latter case, it seemed to be trying to require files relating to a Posts model that doesn't exist under app.
• Therefore the only testing I've done around this change is to check that mix compile still succeeds.

I hope this is OK.

Cheers

Dan

[antonmi]

Thanks, Dan!