sql: fix privilege errors

Via: pingcap/docs-cn#745

sql/privilege.md

@@ -208,13 +208,13 @@ To be more precise, you can check the privilege information in the `Grant` table
 1. Check if `test@%` has global `Insert` privilege:
 
     ```sql
-    select Insert from mysql.user where user='test' and host='%';
+    select Insert_priv from mysql.user where user='test' and host='%';
     ```
 
 2. If not, check if `test@%` has database-level `Insert` privilege at `db1`:
 
     ```sql
-    select Insert from mysql.db where user='test' and host='%';
+    select Insert_priv from mysql.db where user='test' and host='%';
     ```
 
 3. If the result is still empty, check whether `test@%` has table-level `Insert` privilege at `db1.t`: