Update Migrating.md

docs/Migrating.md

   *  Remove the `Brain` GameObjects in the scene
   *  Create new `Brain` Scriptable Objects using `Assets -> Create ->
      ML-Agents`
-  *  Edit their `Brain Paramters` to be the same as the parameters used 
+  *  Edit their `Brain Parameters` to be the same as the parameters used 
      in the `Brain` GameObjects
   *  Agents have a `Brain` field in the Inspector, you need to drag the
   appropriate Brain asset in it.