[doc]Fix autograd.detect_anomaly docs incorrectly formatted (#51335)

Summary:
Fixes #51141

Two bullet points don't render as bullet points.

Before
<img width="657" alt="screenshot before" src="https://user-images.githubusercontent.com/19372617/106240701-125a3080-6248-11eb-9572-f915aa9b72e1.png">

After
<img width="888" alt="screenshot after" src="https://user-images.githubusercontent.com/19372617/106240714-17b77b00-6248-11eb-8e54-51be103639e9.png">

Pull Request resolved: #51335

Reviewed By: izdeby

Differential Revision: D26148582

Pulled By: ezyang

fbshipit-source-id: 5aff6f9bd7affdf13bec965e9bf1a417e5caa88d

torch/autograd/anomaly_mode.py

@@ -7,9 +7,10 @@ class detect_anomaly(object):
     r"""Context-manager that enable anomaly detection for the autograd engine.
 
     This does two things:
+
     - Running the forward pass with detection enabled will allow the backward
-    pass to print the traceback of the forward operation that created the failing
-    backward function.
+      pass to print the traceback of the forward operation that created the failing
+      backward function.
     - Any backward computation that generate "nan" value will raise an error.
 
     .. warning::