minor copy update to LinkedList unit test

in line 21, the it() should say `Should set the head, tail and size of a LinkedList` instead of 
`Should set the data and next field of a node`

https://github.com/knaxus/problem-solving-javascript/blob/ff5e81cbf2cc88ee1a83993c5f18789fe6e1419f/src/_DataStructures_/LinkedList/LinkedList.test.js#L21

i can take this if agreed 