Release process: add missing step.

wiki/src/contribute/release_process.mdwn

@@ -1434,6 +1434,7 @@ this, and skip what does not make sense for a RC.
    appropriately (they should expire after the major release _after_
    the one you're preparing). Look carefully at the output of this command:
 
+        cd "${RELEASE_CHECKOUT:?}" && \
         git checkout "${RELEASE_BRANCH:?}" && \
         for dir in config/APT_snapshots.d vagrant/definitions/tails-builder/config/APT_snapshots.d; do
         (