Fixed typo in `BrainEditor`. (#64)

Replaced `myst` with `must`.

unity-environment/Assets/ML-Agents/Editor/BrainEditor.cs

 		SerializedObject serializedBrain = new SerializedObject (target);
 
 		if (myBrain.transform.parent == null) {
-			EditorGUILayout.HelpBox ("A Brain GameObject myst be a child of an Academy GameObject!", MessageType.Error);
+			EditorGUILayout.HelpBox ("A Brain GameObject must be a child of an Academy GameObject!", MessageType.Error);
 		} else if (myBrain.transform.parent.GetComponent<Academy> () == null) {
 			EditorGUILayout.HelpBox ("The Parent of a Brain must have an Academy Component attached to it!", MessageType.Error);
 		}