Fix default value in documentation (#2452)

Author: JanSellner

pytorch_lightning/trainer/__init__.py

@@ -162,7 +162,7 @@ def forward(self, x):
 Example::
 
     # default used by the Trainer
-    trainer = Trainer(amp_level='O1')
+    trainer = Trainer(amp_level='O2')
 
 auto_scale_batch_size
 ^^^^^^^^^^^^^^^^^^^^^