Fix release link in changelog for v1.21.0

Author: 6543

CHANGELOG.md

@@ -4,7 +4,7 @@ This changelog goes through all the changes that have been made in each release
 without substantial changes to our git log; to see the highlights of what has
 been added to each release, please refer to the [blog](https://blog.gitea.com).
 
-## [1.21.0](https://github.com/go-gitea/gitea/releases/tag/1.21.0) - 2023-11-14
+## [1.21.0](https://github.com/go-gitea/gitea/releases/tag/v1.21.0) - 2023-11-14
 
 * BREAKING
   * Restrict certificate type for builtin SSH server (#26789)