Replace imgur links

[iampi31415]

https://github.com/search?q=repo%3Arust-embedded%2Fdiscovery%20imgur&type=code

The search link above shows where it happens exactly.

One of the reasons is to prevent broken links (currently they serve "content unavailable", rather) for users accessing from the UK (sorry, that's bad).

Of course, one can use a VPN, but seems overkill in this case.

@jamesmunns

See also: https://www.bbc.co.uk/news/articles/c4gzxv5gy3qo

[BartMassey]

Honestly, this is unlikely to get fixed; we have been preparing for some time to archive this version of the Discovery Book anyhow. I'll leave this open for now in case someone wants to do a quick PR of some kind, though.

[iampi31415]

No problem, at least it'll alert others if they get confused by it.