docs: fix image link in README.md

[terriko]

• related: docs: Read the docs build failing #2914

I recently added an image to README.md using a relative link here:

https://github.com/intel/cve-bin-tool/blob/main/README.md?plain=1#L24

It works fine when you view https://github.com/intel/cve-bin-tool/ and get the preview, but we use the README.md file in a few other places including pypi and as part of what we build for Read the Docs, so it would probably be better to use a full url for that image instead.

Probably the right url to use is this "raw" view from github:

https://raw.githubusercontent.com/intel/cve-bin-tool/main/doc/images/cve-bin-tool-workflow-800px.png

This should be a one-line fix and suitable for a new contributor. Please leave this issue for a new contributor taking part in the Open Source Hackathon April 2023.

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• cve-bin-tool's contributor docs
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[stvml]

Hackathon Team 1 would like to work on this issue!