mlaunch docs incorrectly suggests --config can be used for config file

[stennie]

This statement in mlaunch documentation appears to be incorrect:

you can pass in any arbitrary options that a mongos or mongod binary would understand, and mlaunch will pass them on to the correct binary. This includes the -f or --config option to pass on a config file with further options.

At some point the --config option was overloaded to indicate number of config servers, so it looks like -f is the only that will work for passing a config file path.