minor #4682 Fix wrong rounding_mode documentation (bodendorfer-simplethings)

This PR was merged into the 3.x branch.

Discussion
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Fix wrong rounding_mode documentation

Fixes #4681

Commits
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b409348 Fix wrong rounding_mode values

Author: fabpot

doc/filters/format_number.rst

@@ -60,9 +60,9 @@ The list of supported options:
     * ``floor``: Floor rounding
     * ``down``: Rounding towards zero
     * ``up``: Rounding away from zero
-    * ``half_even``: Round halves to the nearest even integer
-    * ``half_up``: Round halves up
-    * ``half_down``: Round halves down
+    * ``halfeven``: Round halves to the nearest even integer
+    * ``halfup``: Round halves up
+    * ``halfdown``: Round halves down
 
     .. code-block:: twig