Added prefix sum array program in .js and updated README.md

[Rishabrp99]

Have you read the Contribution Guidelines?

Yes.

Description

Added prefix sum array program in javascript and updated README.md

Checklist

• [x] I've read the contribution guidelines.
• [x] I've checked the issue list before deciding what to submit.
• [x] I've edited the README.md and link to my code.

Related Issues (Mention Issue no. like #xxx)

Fixes Issue
solved #721