fix: correct format for nullable interval_day parameters (#687)

Fixes a bug in `functions_datetime.yaml` that makes it un-parseable by
substrait-java, introduced in #679. The `?` for nullability must come
before the parameters:
https://substrait.io/types/type_parsing/#type-syntax-parsing

extensions/functions_datetime.yaml

@@ -1047,8 +1047,8 @@ aggregate_functions:
             value: interval_day<P>
         nullability: DECLARED_OUTPUT
         decomposable: MANY
-        intermediate: interval_day<P>?
-        return: interval_day<P>?
+        intermediate: interval_day?<P>
+        return: interval_day?<P>
       - args:
           - name: x
             value: interval_year
@@ -1099,8 +1099,8 @@ aggregate_functions:
             value: interval_day<P>
         nullability: DECLARED_OUTPUT
         decomposable: MANY
-        intermediate: interval_day<P>?
-        return: interval_day<P>?
+        intermediate: interval_day?<P>
+        return: interval_day?<P>
       - args:
           - name: x
             value: interval_year