Fix incorrect arguments in waterworld docstring example (#1058)

pettingzoo/sisl/waterworld/waterworld.py

@@ -83,7 +83,7 @@
 ``` python
 waterworld_v4.env(n_pursuers=5, n_evaders=5, n_poisons=10, n_coop=2, n_sensors=20,
 sensor_range=0.2,radius=0.015, obstacle_radius=0.2,
-obstacle_coord=(0.5, 0.5), pursuer_max_accel=0.01, evader_speed=0.01,
+obstacle_coord=[(0.5, 0.5)], pursuer_max_accel=0.01, evader_speed=0.01,
 poison_speed=0.01, poison_reward=-1.0, food_reward=10.0, encounter_reward=0.01,
 thrust_penalty=-0.5, local_ratio=1.0, speed_features=True, max_cycles=500)
 ```