Update TDS icon URL in README

[ddirks]

Use assets.unidata rather than www.unidata

[lesserwhirls]

Any chance we could get a dark mode friendly version of the icon? Then we could use the supported:

<picture>
  <source media="(prefers-color-scheme: dark)" srcset="dark-mode-image.png">
  <source media="(prefers-color-scheme: light)" srcset="light-mode-image.png">
  <img alt="Fallback image description" src="default-image.png">
</picture>

That said, happy to update it as is!

[ddirks]

Alas, I do not currently have dark mode versions of the software package logos. (I do have dark mode versions of the NSF Unidata logo lockup.)

[lesserwhirls]

No worries - let's go ahead and get this in. Thanks Doug!