Certificate designation is incorrect for --sslPEMKeyFile option

Located on all v2.x docs, for the Connecting a client to mongosqld.

The text describing the use of the --sslPEMKeyFile option was copied from the previous section, however the role of the certificate is reversed and the description is now misleading.

I would recommend replacing "client certificate" with "server certificate" and "mysql_client.pem" with either "mysqld_server.pem" or "mongosqld_server.pem".