Update discovery.rst to link Poetry website on mention.

For folks who aren't super familiar with python, might save them some time.

docs/discovery.rst

@@ -14,7 +14,7 @@ You can install the most recent release using pip:
     pip install python-miio
 
 
-Alternatively, you can clone this repository and use poetry to install the current master:
+Alternatively, you can clone this repository and use `poetry <https://python-poetry.org>`_ to install the current master:
 
 .. code-block:: console