Fix the prepend method for the LinkedList

[m-maksyutin]

Hi, this is a small fix for the following issue:
If you will try to prepend an item to the empty list, the list will have wrong tail.

[codecov-io]

Codecov Report

Merging #47 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #47   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          83     83           
  Lines        1827   1830    +3     
  Branches      322    323    +1     
=====================================
+ Hits         1827   1830    +3

Impacted Files	Coverage Δ
src/data-structures/linked-list/LinkedList.js	100% <100%> (ø)	⬆️

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[trekhleb]

Cool! Thank you for the fix @m-maksyutin