Update a few distro codenames: Jessie → Stretch (refs: #12152)

wiki/src/contribute/release_process/tails-installer.mdwn

@@ -50,7 +50,7 @@ targeted at current Tails, as said above. More specifically:
   in practice this is something like `debian/5.3+dfsg-1`.
 * The `debian/$codename-backports` branch is used to prepare packages
   that we upload to the official backports repository for Debian `$codename`.
-  E.g. here we want to have `debian/jessie-backports` soon after the initially
+  E.g. here we want to have `debian/stretch-backports` soon after the initially
   uploaded package reaches Debian testing. The tags on this branch are
   also called `debian/$package_version`. In practice this is something
   like `debian/5.3+dfsg-1~bpo8+1`.
@@ -292,7 +292,7 @@ You'll need to configure the dput tool to upload to the PPA and put into
 
 * checkout the `debian/sid` branch
   to build a package for the next Ubuntu release or checkout the
-  `debian/jessie-backports` branch to build a package for the
+  `debian/stretch-backports` branch to build a package for the
   current Ubuntu version or current LTS.
 * bump version to `5.3+dfsg-0ubuntu1~$codename` using `dch -i`
   where `$codename` is the name of the target Ubuntu distribution.