docs: formatting fix on delete keyword in mail section (#444)

docs/source/usage.md

@@ -139,7 +139,7 @@ The full set of configuration options are:
   - `watch` - bool: Use the IMAP `IDLE` command to process
   - messages as they arrive or poll MS Graph for new messages
   - `delete` - bool: Delete messages after processing them,
-  - instead of archiving them
+      instead of archiving them
   - `test` - bool: Do not move or delete messages
   - `batch_size` - int: Number of messages to read and process
       before saving. Default `10`. Use `0` for no limit.