Minor fix to link to GAIL reward signal doc (#2435)

docs/Training-Imitation-Learning.md

 The ML-Agents toolkit provides several ways to learn from demonstrations.
 
 * To train using GAIL (Generative Adversarial Imitaiton Learning) you can add the
-  [GAIL reward signal](Reward-Signals.md#the-gail-reward-signal). GAIL can be
+  [GAIL reward signal](Reward-Signals.md#gail-reward-signal). GAIL can be
   used with or without environment rewards, and works well when there are a limited
   number of demonstrations.
 * To help bootstrap reinforcement learning, you can enable