Backend: Update stock amount

[GreeshmaRahool]

Description

Update stock_amout field in products table after payment.

Fixes #142

How to test?

localhost:http://localhost:3000/api/documentation

Checklist

• [X] I have performed a self-review of my own code
• [X] I have followed the name conventions for CSS Classnames and filenames, Components names and filenames, Style filenames, if you are in doubt check the the project README.MD and here https://github.com/HackYourFuture-CPH/curriculum/blob/master/review/review-checklist.md
• [X] I have commented my code, particularly in hard-to-understand areas, if you code was simple enough mark the box anyway
• [X] I have made corresponding changes to the documentation, if you code was simple enough mark the box anyway
• [X] This PR is ready to be merged and not breaking any other functionality

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