Make the digit_frequency.md correct

[imdebamrita]

Make the digit_frequency.md correct

References

https://www.hackerrank.com/challenges/frequency-of-digits-1/problem?isFullScreen=true

Checks

• [x] Tested Changes.
• [ ] Stakeholder Approval.
• [x] My pull request has a descriptive title (not a vague title like "Update index.md").
• [x] My pull request targets the master branch of the repository.
• [x] My code follows the established code style of the repository.
• [ ] I added tests for the changes I made (if applicable).
• [ ] I added or uploaded documentations (if applicable).
• [x] I tried running the project locally and verified that there are no visible errors.

[soham4abc]

LGTM