Hii Aditi,

First of all, while appearing for any examination, you should be very attentive while filling the details in an answer sheet because answer sheets of many candidates get rejected just because of these mistakes. But regarding your particular mistake, you will not face any problem for not writing the word "only" as you have written rest of your roll no in words correctly. But make sure do not make any such mistakes in the future. Also just to be on the safer side, you could send an email to cbse regarding your concern.