Fix name of `requirements.txt` file

[nomicode]

The readme.md file instructs you to run:

pip3 install -r requirements.txt

When you do this, Python returns the following error:

ERROR: Could not open requirements file: [Errno 2] No such file or directory: 'requirements.txt'

This PR fixes the the issue.

[nomicode]

@pvnieo please could you give this PR a review and merge?

[pvnieo]

thank you for noticing, issue fixed!