README.rst: Fix install with circup

Replace nunchuk with adafruit_nunchuk in command circup install
to avoid the following issue:

Searching for dependencies for: ['nunchuk']
WARNING:
	nunchuk is not a known CircuitPython library.
Ready to install: []

Signed-off-by: Leon Anavi <[email protected]>

Author: leon-anavi

README.rst

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