Update persistence.rst

Fix typo: an -> a

docs/topics/persistence.rst

@@ -114,7 +114,7 @@ An **aggregate sequence** is a sequence of stored events for
 an aggregate. The originator version number of the event determines
 its position in its sequence.
 
-An **event notification** is a stored event that also has an notification ID.
+An **event notification** is a stored event that also has a notification ID.
 The notification ID identifies the position of a the event in this sequence.
 
 An **application sequence** is a sequence of event notifications