Enhance MySQL template documentation

Steps to reproduce:

1. Use https://github.com/zabbix/zabbix/tree/master/templates/db/mysql_agent
2. Use password containing for example [email protected]# symbols and follow the configuration example:

   [client]
   user=zbx_monitor
   password=<password>
   

Result:
Incorrect credentials.

Expected:
Example should mention about quoting passwords.

[client]
user=zbx_monitor
password='<password>'