Use the correct variable.

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

@@ -31,8 +31,8 @@ Update POT and PO files
 =======================
 
     git checkout master && \
-    "${RELEASE_CHECKOUT:?}"/import-translations && \
-    "${RELEASE_CHECKOUT:?}"/submodules/jenkins-tools/slaves/check_po && \
+    "${TAILS_GIT_CHECKOUT:?}"/import-translations && \
+    "${TAILS_GIT_CHECKOUT:?}"/submodules/jenkins-tools/slaves/check_po && \
     git add po && \
     git commit -m 'Update POT and PO files, pull updated translations from Transifex.'