fix rounding example (#2238)

Author: lindseymoore

source/reference/operator/aggregation/round.txt

@@ -85,7 +85,7 @@ Definition
             rounds using the first digit to the right of the decimal and
             returns rounded integer value.
 
-            For example, ``$round : [1234.5678, 0]`` returns ``1234``.
+            For example, ``$round : [1234.5678, 0]`` returns ``1235``.
 
 Behavior
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