Associate Binder with a tag (soon to exist), rather than the main bra…

…nch, so it will be more stable.

README.md

@@ -20,7 +20,7 @@ The exercises you should run are on Binder:
 
 <br>
 <p align="center">
-  <a href="https://mybinder.org/v2/gh/jpivarski-talks/2021-02-03-pyhep-numba-tutorial/main?urlpath=lab/tree/tutorial.ipynb">
+  <a href="https://mybinder.org/v2/gh/jpivarski-talks/2021-02-03-pyhep-numba-tutorial/v1.0?urlpath=lab/tree/tutorial.ipynb">
     <img src="https://mybinder.org/badge_logo.svg" alt="Launch Binder" height="40">
   </a>
 <br>