DOCSP-20267 fix desciption under behavior (#2029)

Author: davidhou17

source/reference/operator/aggregation/last-array-element.txt

@@ -57,7 +57,7 @@ Valid operand for :expression:`$last` must resolve to an array, null,
 or missing.
 
 - If the operand resolves to a non-empty array, :expression:`$last`
-  returns the first element in the array:
+  returns the last element in the array:
 
 - If the operand resolves to an empty array ``[]``,
   :expression:`$last` does not return a value.