fixed the merge implementation in MergeSort

[guyariely]

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Describe your change:

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new JavaScript files are placed inside an existing directory.
• All filenames should use the UpperCamelCase (PascalCase) style. There should be no spaces in filenames.
  Example:UserProfile.js is allowed but userprofile.js,Userprofile.js,user-Profile.js,userProfile.js are not
• All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[guyariely]

This is an update on #579.

[pomkarnath98]

Thanks for your contribution @guyariely.
But even after #579, you didn't rectify your mistakes. Once again mentioning, try to use let and const instead of using var and commit.
Otherwise, the code is working fine.

[guyariely]

@pomkarnath98 there are no var in my commit ... it only includes let and const.

[pomkarnath98]

It's not in your commit but in your file.

[guyariely]

I updated the rest of the file to only include let / const.