docs: fix typo in v20 release notes

It makes no sense to wait for it to stabilize on a version released
months previously in the previous major release cycle.

This was probably intended to be recommending the first bugfix release
of the current major.minor release cycle.

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4106>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4106>

docs/relnotes/20.0.0.html

@@ -20,7 +20,7 @@
 <p>
     Mesa 20.0.0 is a new development release. People who are concerned
     with stability and reliability should stick with a previous release or
-    wait for Mesa 19.3.1.
+    wait for Mesa 20.0.1.
 </p>
 <p>
 Mesa 20.0.0 implements the OpenGL 4.6 API, but the version reported by