Update Cells prefix routing spec

What
---
Specify that the prefix needs to be organization bound rather then cell
boud.

Why
---
For Personal Access Tokens we need to be able to move an organization
from 1 cell to another without breaking the token.

Reference: https://gitlab.com/gitlab-org/gitlab/-/issues/442344


Signed-off-by: Steve Xuereb <sxuereb@gitlab.com>