fixed typo: contraint -> constraint

Found a typo of the word constraint in a public method.

`- (NSLayoutConstraint *)topContraintForView:(UIView *)view;`

https://github.com/orta/ORStackView/blob/master/Classes/ios/ORStackView.h#L57
https://github.com/orta/ORStackView/blob/master/Classes/ios/ORStackView.m#L233