bin/gen_release_notes.py: fix conditional of bugfix

Previously this would result in the .0 warning be generated for .z > 0
and the .z == 0 would get the other message.

Fixes: 86079447da
       ("scripts: Add a gen_release_notes.py script")
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>
(cherry picked from commit 69f540c017)

bin/gen_release_notes.py

@@ -57,7 +57,7 @@ TEMPLATE = Template(textwrap.dedent("""\
     <h1>Mesa ${next_version} Release Notes / ${today}</h1>
 
     <p>
-    %if bugfix:
+    %if not bugfix:
         Mesa ${next_version} is a new development release. People who are concerned
         with stability and reliability should stick with a previous release or
         wait for Mesa ${version[:-1]}1.