chore: Updated description of decrypt() usage in src/aws_encryption_sdk/__init__.py

[RitvikKapila]

Issue #, if available:

Description of changes: Updated description of decrypt() usage

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Check any applicable:

• [ ] Were any files moved? Moving files changes their URL, which breaks all hyperlinks to the files.