Update docs/src/further_topics/s3_io.rst

Co-authored-by: Chris Bunney <48915820+ukmo-ccbunney@users.noreply.github.com>

Author: pp-mo

docs/src/further_topics/s3_io.rst

@@ -154,7 +154,7 @@ the objects have been closed and there are no terminals open in that directory.
 
 .. note::
 
-    The ``umount`` is a standard Unix command.  It may not always succeed, in which case
+    The ``umount`` command is a standard Unix command.  It may not always succeed, in which case
     some kind of retry may be needed -- see detail notes below.
 
     The mount created will not survive a system reboot, nor does it function correctly