sql-statement-begin: fix a typo (pingcap#3823)

* sql: fix typo of `Begin` statement

* Update sql-statement-begin.md

Co-authored-by: Lilian Lee <[email protected]>

Author: Joyinqin

sql-statements/sql-statement-begin.md

@@ -34,12 +34,12 @@ Query OK, 0 rows affected (0.01 sec)
 
 ## MySQL compatibility
 
-TiDB supports the syntax extension of `BEGIN PESSMISTIC` or `BEGIN OPTIMISTIC`. This enables you to override the default transactional model for your transaction.
+TiDB supports the syntax extension of `BEGIN PESSIMISTIC` or `BEGIN OPTIMISTIC`. This enables you to override the default transactional model for your transaction.
 
 ## See also
 
 * [COMMIT](/sql-statements/sql-statement-commit.md)
 * [ROLLBACK](/sql-statements/sql-statement-rollback.md)
 * [START TRANSACTION](/sql-statements/sql-statement-start-transaction.md)
 * [TiDB optimistic transaction model](/optimistic-transaction.md)
-* [TiDB pessimistic transaction model](/pessimistic-transaction.md)
+* [TiDB pessimistic transaction model](/pessimistic-transaction.md)