Fix typo in checkpoint docs (#59646)
Summary:
This small typo causing this valuable piece of information to be excluded from the docs.
<img width="876" alt="image" src="https://user-images.githubusercontent.com/8812459/121240517-47f2d400-c84f-11eb-9288-23c551c1591a.png">
The last "warning" is missing a second ":", so it doesn't render in the docs {emoji:1f447}
<img width="875" alt="image" src="https://user-images.githubusercontent.com/8812459/121240467-39a4b800-c84f-11eb-9dd6-ec26754c43d3.png">
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59646
Reviewed By: mruberry
Differential Revision: D28972541
Pulled By: jbschlosser
fbshipit-source-id: d10c6688d8db4d4ec4b02858a4c7b352365219c0
