Improve phrasing.

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

@@ -286,7 +286,7 @@ postpone its expiration date; i.e. we don't differentiate "keep
 a given snapshot around" from "keep a given snapshot usable".
 
 See [[above|time-based_snapshots#bump-expiration-date]] for how we
-can manage `Valid-Until` manually, whenever needed.
+manage `Valid-Until` manually.
 
 One advantage of this design is that we don't have to regularly update
 `Valid-Until` fields, and the corresponding signatures: we only do
@@ -299,8 +299,7 @@ In practice, the main use case for keeping a given time-based APT
 repository snapshot around and valid is when it's being used by
 a release branch:
 
- - `testing`: while it's frozen, that is during 5-10 days most of the
-   time;
+ - `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
    decide to manually point `stable` to a different set of snapshots,