fix apex docstring

Author: awaelchli

pytorch_lightning/plugins/legacy/apex.py

@@ -107,7 +107,7 @@ def clip_gradients(self, grad_clip_val: Union[int, float], optimizer: Optimizer,
             grad_clip_val: Maximum norm of gradients.
             optimizer: Optimizer with gradients that will be clipped.
             norm_type: (float or int): type of the used p-norm. Can be ``'inf'`` for
-            infinity norm.
+                infinity norm.
         """
         model = self.trainer.get_model()
         parameters = model.parameters()