fix binder url (#46)

Author: bjlittle

README.md

@@ -29,7 +29,7 @@
     <img src="https://img.shields.io/github/commits-since/scitools/tephi/latest/master"
         alt="GitHub commits since tagged version" />
 </a>
-<a href="https://mybinder.org/v2/gh/SciTools/tephi/master?filepath=index.ipyn">
+<a href="https://mybinder.org/v2/gh/SciTools/tephi/master?filepath=index.ipynb">
     <img src="https://mybinder.org/badge_logo.svg"
         alt="Launch MyBinderk" />
 </a>