nf-token.sol: APPROWED => APPROVED typo

[Sam Wood]

APPROWED is used several places in nf-token.sol. I assume this is a typo and should be renamed to APPROVED?

[Tadej Vengust]

#247

[Kristijan Sedlak]

@sam-at-luther we noticed that your contribution is eligible to receive a T-shirt. Please send an email with your details (size, name, address) to [e-mail].