chore(docs): Update cover image (#16)

Author: m1so

README.md

@@ -1,6 +1,6 @@
 <div align="center">
 
-![Deepnote Toolkit cover image](./docs/_assets/deepnote-toolkit-cover-image.png)
+![Deepnote Toolkit cover image](https://landing-assets.deepnote.com/deepnote-toolkit-cover-image.png)
 
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