fix process sync example in tutorial

Author: alimanfoo

docs/tutorial.rst

@@ -342,8 +342,8 @@ This array is safe to read or write within a multi-threaded program.
 Zarr also provides support for process synchronization via file locking,
 provided that all processes have access to a shared file system. E.g.::
 
-    >>> synchronizer = zarr.ProcessSynchronizer('example.zarr')
-    >>> z = zarr.open_array('example.zarr', mode='w', shape=(10000, 10000),
+    >>> synchronizer = zarr.ProcessSynchronizer('example.sync')
+    >>> z = zarr.open_array('example', mode='w', shape=(10000, 10000),
     ...                     chunks=(1000, 1000), dtype='i4',
     ...                     synchronizer=synchronizer)
     >>> z

zarr/sync.py

@@ -33,6 +33,7 @@ class ProcessSynchronizer(object):
     ----------
     path : string
         Path to a directory on a file system that is shared by all processes.
+        N.B., this should be a *different* path to where you store the array.
 
     """  # flake8: noqa
 
@@ -42,7 +43,7 @@ def __init__(self, path):
     def __getitem__(self, item):
         import fasteners
         lock = fasteners.InterProcessLock(
-            os.path.join(self.path, '%s.lock' % item)
+            os.path.join(self.path, item)
         )
         return lock