docs(readme): use external URL for video cover
@@ -11,7 +11,7 @@ As universal function approximators, artificial neural networks have been shown
<p align='center'>
<a href='https://youtu.be/VDLwyFD-sXQ'>
- <img src="resources/watermark-cover.jpg" width="80%">
+ <img src="https://raw.githubusercontent.com/NeuroDiffGym/neurodiffeq/master/resources/watermark-cover.jpg" width="80%">
</a>
</p>
