Fix typo in comment

[xiaomingfu]

Simple change to fix the typo in a comment in RedBlackTree.js

This is to address issue #528

[codecov]

Codecov Report

Merging #540 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #540   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          150       150           
  Lines         2627      2627           
  Branches       439       439           
=========================================
  Hits          2627      2627           

Impacted Files	Coverage Δ
...ata-structures/tree/red-black-tree/RedBlackTree.js	100.00% <ø> (ø)

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