Fix typo in source/reference/operator/all.txt

Author: joemcbride

source/reference/operator/all.txt

@@ -20,7 +20,7 @@ $all
 
    This query selects all documents in the ``inventory`` collection
    where the ``tags`` field contains an array with the elements,
-   ``appliances``, ``school``, and ``technology``.
+   ``appliances``, ``school``, and ``book``.
 
    Therefore, the above query will match documents in the ``inventory``
    collection that have a ``tags`` field that hold *either* of the