DOC: Fix Google Colab badge

Can't do `<img src=SVG>`.

Author: kernc

backtesting/__init__.py

@@ -15,7 +15,7 @@
 
 These tutorials are also available as live Jupyter notebooks:
 [![Binder](https://mybinder.org/badge_logo.svg)][binder]
-[![Google Colab](https://colab.research.google.com/assets/colab-badge.svg)][colab]
+[![Google Colab](https://colab.research.google.com/assets/colab-badge.png)][colab]
 <br>In Colab, you might have to `!pip install backtesting`.
 
 [binder]: \