Improve phrasing.

This is not hypothetical, it's how we've been doing it for a while now.

wiki/src/contribute/APT_repository/time-based_snapshots.mdwn

@@ -303,7 +303,7 @@ a release branch:
  - `stable`: that's a corner case, since `stable` generally uses the
    set of snapshots frozen during the last Tails freeze; if and when we
    decide to manually point `stable` to a different set of snapshots,
-   then we can as well deal with `Valid-Until` manually.
+   then we bump `Valid-Until` manually.
 
 In passing, note that we ship an empty `/var/cache/apt/lists/` in the
 ISO ⇒ modifying `Release` and `Release.gpg` files on our APT