fixed hours bug (#3352)

Author: gmiller-mdb

source/reference/operator/aggregation/dateTrunc.txt

@@ -182,12 +182,12 @@ Behavior
 For example:
 
 - If :ref:`binSize <dateTrunc-binSize>` is ``1`` and :ref:`unit
-  <dateTrunc-unit>` is ``hours``, the time period is one hour. For the
+  <dateTrunc-unit>` is ``hour``, the time period is one hour. For the
   :ref:`date <dateTrunc-date>` ``2021-03-20T11:30:05Z``,
   :expression:`$dateTrunc` returns ``2021-03-20T11:00:00Z``.
 
 - If :ref:`binSize <dateTrunc-binSize>` is ``2`` and :ref:`unit
-  <dateTrunc-unit>` is ``hours``, the time period is two hours. For the
+  <dateTrunc-unit>` is ``hour``, the time period is two hours. For the
   :ref:`date <dateTrunc-date>` ``2021-03-20T11:30:05Z``,
   :expression:`$dateTrunc` returns ``2021-03-20T10:00:00Z``.