list.add seems to have a minor mistake

[Rui-L]

I think this should be if l.Len() == index {
https://github.com/arnauddri/algorithms/blob/master/data-structures/linked-list/linked_list.go#L82

[shengzhch]

if i.Len() <= index better?

[Mohammadjafari80]

I actually corrected it. here is my pull request #21