I don't see anything iphone related at that link, but maybe it's just me. I don't have a youtube account or allow their cookies.
Anyway, it's somewhere in the /System/Library/CoreServices/SpringBoard.app/English.lproj/SpringBoard.strings file. There are several lines that say "slide to unlock" and you can change whatever is between the quotation marks. It should be self-explanatory when you look at the file but I don't know the exact line to change because I changed all mine to ""