Correct information on readme.md on where to find customer keys

[dsottimano]

Change point 7 : Configura el plugin ingresando el P_CUST_ID_CLIENTE, PUBLIC_KEY y P_KEY, los puedes ver en tu panel de clientes.

This is the correct link:
https://dashboard.epayco.com/configuration

Please also mention that it is under the accordion "Llaves secretas"