fix windowizable predicate

Author: TrevorBergeron

bigframes/core/expression_factoring.py

@@ -134,7 +134,7 @@ def windowize(
 ) -> expression.Expression:
     def windowize_local(expr: expression.Expression):
         if isinstance(expr, agg_expressions.Aggregation):
-            if expr.op.can_be_windowized:
+            if not expr.op.can_be_windowized:
                 raise ValueError(f"Op: {expr.op} cannot be windowized.")
             return agg_expressions.WindowExpression(expr, window)
         if isinstance(expr, agg_expressions.WindowExpression):