[doc] Minor fix

Summary:
Don't interrupt user's flow that they may need to install `tabulate`.
Also, show them the actual command that they need to run to install
`tabulate`.

Test Plan:
Look at rendered doc.

Author: c-p-i-o

prototype_source/flight_recorder_tutorial.rst

@@ -199,6 +199,14 @@ Caveat: tabulate module is needed, so you might need pip install it first.
   python fr_trace.py <dump dir containing trace files> -j [--selected-ranks i j k ...] [--pg-filters tp dp]
   torchfrtrace <dump dir containing trace files> -j [--selected-ranks i j k ...] [--pg-filters 0 2]
 
+
+You may need to install the `tabulate` python module if the command does not work.
+
+.. code:: shell
+
+  pip install tabulate
+
+
 Conclusion
 ----------
 In this tutorial, we have learned about a new PyTorch diagnostic tool called Flight Recorder.