Fix typo in comment (#690)

Author: soumith

reinforcement_learning/actor_critic.py

@@ -63,7 +63,7 @@ def forward(self, x):
         # critic: evaluates being in the state s_t
         state_values = self.value_head(x)
 
-        # return values for both actor and critic as a tupel of 2 values:
+        # return values for both actor and critic as a tuple of 2 values:
         # 1. a list with the probability of each action over the action space
         # 2. the value from state s_t 
         return action_prob, state_values