Fixed wrong on_fit_start callback reference in documentation (#7001)

Co-authored-by: Daniele Acquaviva <danieleacquaviva@Danieles-MacBook-Pro.local>

Author: dacquaviva

docs/source/extensions/callbacks.rst

@@ -167,7 +167,7 @@ on_init_end
 on_fit_start
 ^^^^^^^^^^^^
 
-.. automethod:: pytorch_lightning.callbacks.Callback.on_save_checkpoint
+.. automethod:: pytorch_lightning.callbacks.Callback.on_fit_start
     :noindex:
 
 on_fit_end