refactor: better comment

Signed-off-by: hi-rustin <rustin.liu@gmail.com>

pkg/planner/core/task.go

@@ -1891,7 +1891,7 @@ func RemoveUnnecessaryFirstRow(
 					// the firstrow in root task can not be removed.
 					break
 				}
-				// Skip it if it is constant.
+				// Skip if it's a constant.
 				// For SELECT DISTINCT SQRT(1) FROM t.
 				// We shouldn't remove the firstrow(SQRT(1)).
 				if _, ok := gbyExpr.(*expression.Constant); ok {