correct url

Author: rflamary

README.md

@@ -153,7 +153,7 @@ Every contribution is welcome and should respect the [contribution guidelines](C
 
 You can ask questions and join the development discussion:
 
-* On the [POT Slack channel](pot-toolbox.slack.com)
+* On the [POT Slack channel](https://pot-toolbox.slack.com)
 * On the POT [mailing list](https://mail.python.org/mm3/mailman3/lists/pot.python.org/)