Update Embassy Clock showcase to have an image

[jamesmunns]

CC @CarlKCarlK, it seems that the video tag wants an actual video link to embed, we should probably swap to an image demo, or make the showcase smart enough to understand youtube embeds.

This is a follow up to #36

[jamesmunns]

https://github.com/rust-embedded/showcase/pull/36/files#diff-093f056f1d238f3fb59742e58ef57a2a0d056e07e42e18a72b451da2801a489eR84-R87

[BartMassey]

Thanks for taking care of this!