post_version.py: fix relnotes links

Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5928>
diff --git a/bin/post_version.py b/bin/post_version.py
index 328a48b..359cbd9 100755
--- a/bin/post_version.py
+++ b/bin/post_version.py
@@ -34,10 +34,14 @@
 
     new_relnotes = []
     first_list = True
+    second_list = True
     for line in relnotes:
         if first_list and line.startswith('-'):
             first_list = False
-            new_relnotes.append(f'- `{version} release notes <relnotes/{version}.rst>`__\n')
+            new_relnotes.append(f'-  :doc:`{version} release notes <relnotes/{version}>`\n')
+        if not first_list and second_list and line.startswith('   relnotes/'):
+            second_list = False
+            new_relnotes.append(f'   relnotes/{version}\n')
         new_relnotes.append(line)
 
     with open(p, 'w') as f: