graphstrategy: fix error message about multiple root states

root_states already contains all states without dependencies as
strings. Joining only the first element of each string results in the
first character of each state being printed. Fix this by joining
root_states directly without a list comprehension.

Signed-off-by: Bastian Krause <bst@pengutronix.de>

labgrid/strategy/graphstrategy.py

@@ -81,7 +81,7 @@ def __attrs_post_init__(self):
         if len(root_states) > 1:
             raise InvalidGraphStrategyError(
                 'Only one root state supported. Defined root states: {}'.format(  # NOQA
-                    ', '.join([i[0] for i in root_states]),
+                    ', '.join(root_states),
                 )
             )