Update mysql `connection_string` config description in doc

[xring]

Ref to https://opendal.apache.org/docs/services/mysql

Configuration
root: Set the working directory of OpenDAL
connection_string: Set the connection string of postgres server
table: Set the table of mysql
key_field: Set the key field of mysql
value_field: Set the value field of mysql

description for connection_string should be mysql server not postgres server