fix group documentation indentation

Author: brokkoli71

src/zarr/core/group.py

@@ -1738,6 +1738,7 @@ def nmembers(self, max_depth: int | None = 0) -> int:
             default, (``max_depth=0``) only immediate children are included. Set
             ``max_depth=None`` to include all nodes, and some positive integer
             to consider children within that many levels of the root Group.
+
         Returns
         -------
         count : int