Improve err message grammar (#16566)

Author: findepi

datafusion/sql/src/expr/function.rs

@@ -223,8 +223,8 @@ impl<S: ContextProvider> SqlToRel<'_, S> {
         } = function_args;
 
         if over.is_some() && !within_group.is_empty() {
-            return plan_err!("OVER and WITHIN GROUP clause are can not be used together. \
-                OVER is for window function, whereas WITHIN GROUP is for ordered set aggregate function");
+            return plan_err!("OVER and WITHIN GROUP clause cannot be used together. \
+                OVER is for window functions, whereas WITHIN GROUP is for ordered set aggregate functions");
         }
 
         // If function is a window function (it has an OVER clause),

datafusion/sqllogictest/test_files/aggregate.slt

@@ -170,7 +170,7 @@ statement error DataFusion error: This feature is not implemented: Only a single
 SELECT approx_percentile_cont(0.95) WITHIN GROUP (ORDER BY c5, c12) FROM aggregate_test_100
 
 # Not supported over sliding windows
-query error DataFusion error: Error during planning: OVER and WITHIN GROUP clause are can not be used together. OVER is for window function, whereas WITHIN GROUP is for ordered set aggregate function
+query error DataFusion error: Error during planning: OVER and WITHIN GROUP clause cannot be used together. OVER is for window functions, whereas WITHIN GROUP is for ordered set aggregate functions
 SELECT approx_percentile_cont(0.5)
 WITHIN GROUP (ORDER BY c3)
 OVER (ROWS BETWEEN 4 PRECEDING AND CURRENT ROW)