docs: use .png extenstion instead of .pdf in screenshot guide

[withden]

What kind of change does this PR introduce?
There is a .pdf extension for screenshot docs (example). This PR request changes that extension to .png (As screenshots are always image)

Did you add tests for your changes?
As the changes in docs, it's not needed to testing

[google-cla]

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[conventional-commit-lint-gcf]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot
https://conventionalcommits.org/