Async's read (#36422)

Author: vincenttran-msft

sdk/storage/azure-storage-blob/azure/storage/blob/aio/_download_async.py

@@ -468,7 +468,7 @@ def read(self, size: int = -1) -> T:
         ...
 
     @overload
-    def read(self, *, chars: Optional[int] = None) -> T:
+    def read(self, *, chars: Optional[int] = None) -> str:
         ...
 
     # pylint: disable-next=too-many-statements,too-many-branches