[gnomemm-website] README.md: Fix error in markdown

[Kjell Ahlstedt <kjellahl src gnome org>]

commit ab9ddd664b777b1bc162f16791ef81f61d1ef3c4
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Thu Jan 27 14:50:27 2022 +0100

    README.md: Fix error in markdown

 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/README.md b/README.md
index 42cbe14..44ee18e 100644
--- a/README.md
+++ b/README.md
@@ -35,7 +35,7 @@ The website can be built with Meson or Autotools.
   At the time of writing (2022-01-27) the `publish` target is disabled.
   You only get instructions how to enable it.
 
-# Building with Autotools
+## Building with Autotools
 
 - Configure, build in the source directory:
   ```sh