added string formatting

[mananuppadhyay]

Please fill in this pull request template before submitting

1. This pull request resolves #102

2. Description

Created a new folder named string formatting and added a tutorial for f-strings in it.

3. Fill in checklist by marking [x]

• [x] I've read the CONTRIBUTING.md
• [x] I was assigned to this issue
• [x] My code is formatted using required linting
• [x] I've run my code locally and checked it works

[mananuppadhyay]

Thanks @mananuppadhyay for your contribution ! 🎉 I've requested small changes to improve the PR

hey i committed the changes requested, but the Github checks show errors. i'm not sure how to resolve them. could you help?