To make things even more clear and complicated:
They're not words, they're just bytes. Each zero is a nibble, a hex digit, four bits... So 00 is just the null byte.
In short:
Replace each character in "URL:" with the null character, which has a hex value of 00.
For those looking for a hex editor,
XVI32 does the trick. It's a small, free, simple, fast (all qualities we like around here ;-) hex editor.