Fixed typo in docs (concepts). (#1295)

Author: theodumont

docs/source/concepts.rst

@@ -70,8 +70,8 @@ Argument ``batch`` in ``train_step`` function is user-defined and can contain an
     optimizer_1 = ...
     optimizer_2 = ...
     # ...
-    critertion_1 = ...
-    critertion_2 = ...
+    criterion_1 = ...
+    criterion_2 = ...
     # ...
 
     def train_step(trainer, batch):