docs(README): fix 'Contribution guidelines' link
docs(README): fix 'Contribution guidelines' link

The current link leads to a page 'Building and Testing AngularJS'.
This same link is also included in the 'Building AngularJS' section
of the README where it's more relevant.

ggershoni/angular.js,ggershoni/angular.js,ggershoni/angular.js,ggershoni/angular.js