LockscreenCredential: clarify Effective Java rule

The "Always override hashCode when you override equals" rule in
Effective Java now has different item numbers across editions.
Update the comment to the actual rule used.

Test: new comment reads correctly

Change-Id: Ibdbce008f653f8800acc76176ad06ffe347dc106
Signed-off-by: Julius D'souza <jdsouza@google.com>