doc: specify full main XML filename

For historical reasons, AutoDoc allows omitting the file
extensions, but this is an undocumented feature. So better
not to rely on it.

Author: fingolfin

makedoc.g

@@ -131,7 +131,7 @@ AutoDoc("semigroups", rec(
             \usepackage{a4wide}
             \newcommand{\bbZ}{\mathbb{Z}}
         """),
-        main := "main",
+        main := "main.xml",
         files := Files),
 
     scaffold := rec(