Fixup doctest requires

Author: dstansby

docs/tutorial.rst

@@ -811,7 +811,7 @@ storing an array or group. Here is an example using a Berkeley DB B-tree
 database for storage (requires `bsddb3
 <https://www.jcea.es/programacion/pybsddb.htm>`_ to be installed):
 
-.. doctest-requires::
+.. doctest-requires:: bsddb3
 
     >>> import bsddb3
     >>> store = zarr.DBMStore('data/example.bdb', open=bsddb3.btopen)