Update tutorial.rst (#634)

typo fix

Author: taras-sereda

docs/tutorial.rst

@@ -746,7 +746,7 @@ Python is built with SQLite support)::
 Also added in Zarr version 2.3 are two storage classes for interfacing with server-client
 databases. The :class:`zarr.storage.RedisStore` class interfaces `Redis <https://redis.io/>`_
 (an in memory data structure store), and the :class:`zarr.storage.MongoDB` class interfaces
-with `MongoDB <https://www.mongodb.com/>`_ (an oject oriented NoSQL database). These stores
+with `MongoDB <https://www.mongodb.com/>`_ (an object oriented NoSQL database). These stores
 respectively require the `redis-py <https://redis-py.readthedocs.io>`_ and
 `pymongo <https://api.mongodb.com/python/current/>`_ packages to be installed.