[doc] fix flight recorder tutorial (pytorch#3189)

Summary:
The small example code isn't rendering on the page.
Remove caption as this seems to be problematic.

Test Plan:
Test on sandbox.

Author: c-p-i-o

prototype_source/flight_recorder_tutorial.rst

@@ -214,7 +214,6 @@ For demonstration purposes, we named this program ``crash.py``.
    complexities.
 
 .. code:: python
-  :caption: A crashing example
 
   import torch
   import torch.distributed as dist