Fix broken image paths

[shuuji3]

Description

README.md has three broken images. This is because the image path is not correct. This PR replaces ./chrome-store/ with ./assets/ in those paths.

Type of change

• [x] This change requires a documentation update

How Has This Been Tested?

You can confirm the change to fix images on this forked branch: https://github.com/shuuji3/headless-recorder/tree/fix-image-path

I couldn't pass the npm run test on my environment but it should pass because this PR only changes README.md.

Checklist:

• [x] My code follows the style guidelines of this project. npm run lint passes with no errors.
• [x] I have made corresponding changes to the documentation
• [x] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes. npm run test passes with no errors.

[ianaya89]

@shuuji3 thank you!