filter-repo (README): remove outdated 2.28.0-not-yet-released comment

Signed-off-by: Elijah Newren <newren@gmail.com>

README.md

@@ -430,7 +430,7 @@ has also driven numerous improvements to fast-export and fast-import
 (and occasionally other commands) in core git, based on things
 filter-repo needs to do its work:
 
-  * git-2.28.0 (not yet released)
+  * git-2.28.0
     * [fast-import: add new --date-format=raw-permissive format](
       https://git.kernel.org/pub/scm/git/git.git/commit/?id=d42a2fb72f)
   * git-2.24.0