docs: fix circup install command

prefixes the `circup` install command package with `adafruit_` so it can be installed

Author: imnotjames

README.rst

@@ -84,7 +84,7 @@ following command to install:
 
 .. code-block:: shell
 
-    circup install ducky
+    circup install adafruit_ducky
 
 Or the following command to update an existing version: