Update gdp.py (#386)

Summary:
## Types of changes

- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing functionality to change)
- [ ] Docs change / refactoring / dependency upgrade

## Motivation and Context / Related issue

`PrivacyEngine()` cannot be used properly with "GDP" mechanism, since the mechanism for `GaussianAccountant` returns the string "gpd" instead of "gdp"

## How Has This Been Tested (if it applies)

Ran code before and after correcting the typo- works now.

## Checklist

- [x] The documentation is up-to-date with the changes I made.
- [x] I have read the **CONTRIBUTING** document and completed the CLA (see **CONTRIBUTING**).
- [x] All tests passed, and additional code has been covered with new tests.

Pull Request resolved: #386

Reviewed By: karthikprasad

Differential Revision: D35146020

Pulled By: ffuuugor

fbshipit-source-id: e93e76b5e398eb3966ab5e33f5a696d375b34955

opacus/accountants/gdp.py

@@ -66,4 +66,4 @@ def __len__(self):
 
     @classmethod
     def mechanism(cls) -> str:
-        return "gpd"
+        return "gdp"