docs: update RPi links in docs/linux/external_fuzzing_usb.md

[ajdlinux]

Is your feature request related to a problem? Please describe. Links to Raspberry Pi documentation in docs/linux/external_fuzzing_usb.md are outdated and no longer link to the specific parts of the documentation that is referred to. It also suggests downloading Raspbian Stretch, which is ancient.

Describe the solution you'd like Update the documentation links to match the present documentation structure, and update the download link to a current version of Raspbian.