docs(readme): full urls for image assets
<h1 align="center">
<br>
- <img src="assets/logo.png" alt="logo" width="200">
+ <img src="https://raw.githubusercontent.com/zellij-org/zellij/main/assets/logo.png" alt="logo" width="200">
<br>
Zellij
<br>
</h1>
<p align="center">
- <img src="assets/demo.gif" alt="demo">
+ <img src="https://raw.githubusercontent.com/zellij-org/zellij/main/assets/demo.gif" alt="demo">
</p>
<p align="center">
