You only worked 1 week prior to the resignation, so the most
they deduct from you is 1 day's pay. 
You have to check to see if your employment contract
specifically states 1 week notice. It's best you call Ministry of
Manpower to confirm.
Notice of termination of
contract
10. —(1) Either party to a contract of service may
at any time give to the other party notice of his intention to
terminate the contract of service.
(2) The length of such notice shall be the same for both
employer and employee and shall be determined by any provision made
for the notice in the terms of the contract of service, or, in the
absence of such provision, shall be in accordance with subsection
(3).
(3) The notice to terminate the service of a person who is
employed under a contract of service shall be not less than —
(a) one day’s notice if he has been so employed for less
than 26 weeks;
(b) one week’s notice if he has been so employed for 26 weeks or
more but less than 2 years;
(c) 2 weeks’ notice if he has been so employed for 2 years or
more but less than 5 years; and
(d) 4 weeks’ notice if he has been so employed for 5 years or
more.
(4) This section shall not be taken to prevent either party from
waiving his right to notice on any occasion.
(5) Such notice shall be written and may be given at any time,
and the day on which the notice is given shall be included in the
period of the notice.