Release process: set WEBSITE_RELEASE_BRANCH in a way that works more nicely for RCs.

wiki/src/contribute/release_process.mdwn

@@ -58,7 +58,7 @@ the scripts snippets found on this page:
 * `DIST`: either 'alpha' (for RC:s) or 'stable' (for actual releases)
 * `export DEBEMAIL='tails@boum.org'`
 * `export DEBFULLNAME='Tails developers'`
-* `export WEBSITE_RELEASE_BRANCH="web/release-${VERSION:?}"`
+* `export WEBSITE_RELEASE_BRANCH="web/release-${TAG:?}"`
 
 Pre-freeze
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