Fixing typo

Pretty sure this is a typo.

Author: fattslug

source/reference/operator/aggregation/toUpper.txt

@@ -27,7 +27,7 @@ Definition
    as long as it resolves to a string. For more information on
    expressions, see :ref:`aggregation-expressions`.
 
-   If the argument resolves to null, :expression:`$toLower` returns an
+   If the argument resolves to null, :expression:`$toUpper` returns an
    empty string ``""``.
 
 Behavior