Update exponential_search.md with C code

[Niveshpai]

Fixes #

Tasks

• [x] Updated C code implementation of Exponential Search

Please check that your PR satisfies these

• [x] The contribution is not majorly plagarised.
• [x] I have raised a issue regarding my work.
• [x] I have done appropriate readme updates (if needed).

Note: Please check Allow edits from maintainers. if you would like us to assist in the PR.