navel_piercing -> navel

epic bump

Epic bump indeed, but it's a valid reason for doing so. After looking it over, implication is made and approved.

Note: If there is ever a situation where a character is drawn without a navel but still has a piercing where a navel would normally be, then it can be tagged with belly_piercing instead. So there shouldn't be any issues from that. (Since navels aren't always standard issue for all characters or species.)