I'm not so sure about this. I understand that the official spelling is FiveFingers, but the CamelCase distinguishes the two words in it. Since we don't have uppercase letters in tags, I think five-fingers is more clear.
I want to put @choster´s comment as a votable answer here, because I find it very important:
We should not rename the tag, but delete it! It´s a product name, do you want to get a tag for every product (or even for every toe-sandal)? I think we should, at least in tags, stay absolutely neutral. So we delete product names and put, in replacement for five-fingers, create/use the tag toe-sandals. We can still create a synonym for it, so the individual products get connected to this tag.