Add 'import minigrid' to example in docs/index.md. (#458)

docs/index.md

@@ -27,6 +27,7 @@ The Gymnasium interface allows to initialize and interact with the Minigrid defa
 ```{code-block} python
 
 import gymnasium as gym
+import minigrid
 env = gym.make("MiniGrid-Empty-5x5-v0", render_mode="human")
 observation, info = env.reset(seed=42)
 for _ in range(1000):