Change absolute links to relative links.

The links in the README.md and CONTRIBUTING.md files that point to files in the repo are absolute links rather than being relative links. Please correct them to relative links, taking care of directory levels.
This issue is not related to external links (ie, links that go to external web sites) in those two files.
Example of relative links: CONTRIBUTING.md. [CONTRIBUTING.md](CONTRIBUTING.md) ✔️
Example of absolute links: CONTRIBUTING.md. [CONTRIBUTING.md](https://github.com/HarshKapadia2/git_basics/blob/master/CONTRIBUTING.md) ❌

HarshKapadia2 / git_basics