Fix a typo about enabling tls (#3801)

[ti-srebot]

cherry-pick #3801 to release-4.0

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What is changed, added or deleted? (Required)

I think it should be require x509。

I tested this SQL create user 'u1'@'%' require 509; . The error result returned is as follows

MySQL [(none)]> create user 'u1'@'%'  require 509;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your TiDB version for the right syntax to use line 1 column 33 near "509"

create user 'u1'@'%' require x509; is right .

MySQL [(none)]> create user 'u1'@'%'  require x509;
Query OK, 0 rows affected (0.02 sec)

PS : I just want to try and submit a PR .
😃

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v4.0 (TiDB 4.0 versions)
• v3.1 (TiDB 3.1 versions)
• v3.0 (TiDB 3.0 versions)
• v2.1 (TiDB 2.1 versions)

[lysu]

LGTM

[lilin90]

LGTM