Fix link to Sphinx-Gallery (#486)

The link does not work because Sphinx-Gallery's documentation is no longer hosted on Read the Docs.

Author: Kenso Trabing

README.md

@@ -9,7 +9,7 @@ All the tutorials are now presented as sphinx style documentation at:
 
 # Contributing
 
-We use sphinx-gallery's [notebook styled examples](https://sphinx-gallery.readthedocs.io/en/latest/tutorials/plot_notebook.html#sphx-glr-tutorials-plot-notebook-py) to create the tutorials. Syntax is very simple. In essence, you write a slightly well formatted python file and it shows up as documentation page.
+We use sphinx-gallery's [notebook styled examples](https://sphinx-gallery.github.io/tutorials/plot_notebook.html#sphx-glr-tutorials-plot-notebook-py) to create the tutorials. Syntax is very simple. In essence, you write a slightly well formatted python file and it shows up as documentation page.
 
 Here's how to create a new tutorial:
 1. Create a notebook styled python file. If you want it executed while inserted into documentation, save the file with suffix `tutorial` so that file name is `your_tutorial.py`.