Updated README to show correct usage of Set command

[varunu28]

Issue
Set API doesn't returns 3 values as mentioned in the documentation. It just returns an error instance.

func (tx *Tx) Set(key string, value string) error {
	e := newRecord([]byte(key), []byte(value), StringRecord, StringSet)
	tx.addRecord(e)

	return nil
}

Changes
Updating the documentation to reflect the correct usage of Set API

[arriqaaq]

Thank you for the fix, appreciate it