Update README.md - Add Prefix "camelSalesforce"

The suffix "camelSalesforce" needs to be added as prefix to the properties.
The command should be corrected as:
mvn camel-salesforce:generate -DcamelSalesforce.clientId=<clientid> -DcamelSalesforce.clientSecret=<clientsecret> -DcamelSalesforce.userName=<username> -DcamelSalesforce.password=<password>