readme fix

[talco]

hi
i wanted to use the schema tool to create the user tables in mysql but i got errors.

it succeded only when i used this line.
cake schema create --name users --plugin Users

notice the capitale U in "--plugin Users"
whitout it its not working.

also it works only when you type the command when your location is in the app dir. any other location will not work.

thats all,
thanks for the plugin
Tal