fix condition of Add in Linked List

[Mohammadjafari80]

There was a tiny problem with the condition. when the index is equal to the length of the list we must append it to the tail not when it's equal to length - 1. The reason is that we start counting from zero therefore we should have at least length + 1 spot for inserting a node( counting length from zero). I guess you didn't notice because the tests actually didn't cover it.