You can't correct an entry if you can't find it again. If you are sure that the passenger list you saw was in the right time period, then...
Ellis Island (1892-1924) passenger list indexes are available on at least 3 sites:
The Ellis Island Foundation website includes indexes and images. On the Passenger Record page (i.e. the index entry) and on the manifest navigation page, there is a menu item call "Create an Annotation", which is where corrections and other information can be recorded.
Ancestry's database,"New York, Passenger Lists, 1820-1957" has indexes and images for arrivals before, during and after the Ellis Island period. Corrections to transcription errors and entry errors (for some fields) can be added using the "Add Alternate Info" link on the individual record (index page) or by clicking on a field in the index lines below the image (same as for most databases at Ancestry.com). Or "Leave a Comment."
FamilySearch's index, "New York, Passenger Arrival Lists (Ellis Island), 1892-1924" links to images of arrival lists at the Statue of Liberty-Ellis Island Web site.
No corrections can be made to any FamilySearch indexes at this time. See above for the Ellis Island website.
Generally the Ellis Island index is more accurate than Ancestry's, but your mileage may vary. Also I often use Steve Morse's "Ellis island One-Step" search for even more flexibility.