The IC already has two resistors between output and ground, inside it.
, page 5 shows the diagram.
Could you adjust it? Yeah, you probably could, but it gets complicated, you may push too much current on the ground pin and other stuff.
The datasheet has an adjustable regulator design on page 29, but it's not quite as simple as a couple of resistors, they use an opamp.
Everybody recommends 7805 because it's very cheap and simple to use but nowadays there are much better, and still cheap, components.
funk1980, you have to be aware that this 7805 tipically needs as much as 2v above the output voltage to keep the output smooth, stable and so on.
If you pull 1A, to get stable 5v output, it needs 7v. At less current, it needs less input voltage, but would probably still need at least 1.5v
Depending on how that power supply you have is built, that 7v may hover around 6.8-7.2v, so you have to be careful. 99% you won't have a problem.
But if you can, go for a voltage regulator that's designed to be adjustable from the start and not demand so much voltage above its output setting, as 7805 and 317 do.
For example, I personally like MIC2941 and LT1084/LT1085/LT1085 but they're a bit expensive at $3-4 a piecehttp://uk.farnell.com/micrel-semiconductor/mic2941awt/ic-reg-ldo-1-25a-adj/dp/1556737http://uk.farnell.com/linear-technology/lt1086ct-pbf/v-reg-ldo-adj-1-2-29v-1086-to-220/dp/9559868
MIC2941 has a maximum dropout of 0.35v while LT1086 is guaranteed to be under 1v ... datasheets are there in the links above for both.