Fix doc: the stable branch does not use *tagged* snapshots generally.

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

@@ -301,7 +301,7 @@ a release branch:
 
  - `testing`: while it's frozen, that is for 5-10 days generally;
  - `stable`: that's a corner case, since `stable` generally uses the
-   set of tagged snapshots of the latest Tails release; if and when we
+   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.