examples: Switch ftp.gnu.org to mirror

This should avoid current issues accessing the necessary sources which
are blocking CI currently.

doc/examples/autotools/project.conf

@@ -10,4 +10,4 @@ element-path: elements
 # Define some aliases for the tarballs we download
 aliases:
   alpine: https://gnome7.codethink.co.uk/tarballs/
-  gnu: https://ftp.gnu.org/gnu/automake/
+  gnu: https://ftpmirror.gnu.org/gnu/automake/

doc/examples/junctions/autotools/project.conf

@@ -10,4 +10,4 @@ element-path: elements
 # Define some aliases for the tarballs we download
 aliases:
   alpine: https://gnome7.codethink.co.uk/tarballs/
-  gnu: https://ftp.gnu.org/gnu/automake/
+  gnu: https://ftpmirror.gnu.org/gnu/automake/