The regular way to pluralize any noun is by adding just an s; the apostrophe should only be added to plural s if the word would otherwise become unreadable or exceedingly ambiguous. A good example would be s's (the plural of the letter s: ss would look like an acronym).
The word URLs would seem to be quite clear: *URL's is probably a simple error made by those confusing it with the possessive `s's, which always has an apostrophe (a few pronouns being the exception, like its and hers).