mdbook-graphviz: Add myself as maintainer

Signed-off-by: Matthias Beyer <[e-mail]>

pkgs/tools/text/mdbook-graphviz/default.nix

···
     homepage = "https://github.com/dylanowen/mdbook-graphviz";
     changelog = "https://github.com/dylanowen/mdbook-graphviz/releases/tag/v${version}";
     license = [ licenses.mpl20 ];
-    maintainers = with maintainers; [ lovesegfault ];
+    maintainers = with maintainers; [ lovesegfault matthiasbeyer ];
   };
 }