Update add_links.sh

Ensure add_links.sh uses python3

Author: serban-nicusor-toptal

add_links.sh

@@ -10,9 +10,9 @@ mkdir docs/"$directory"_tmp
 
 (cd docs/"$directory"; tar cf - .) | (cd docs/"$directory"_tmp; tar xvf - )
 
-python link_to_latest.py docs/"$directory"_tmp/stan-users-guide "Stan User's Guide"
-python link_to_latest.py docs/"$directory"_tmp/reference-manual "Stan Reference Manual"
-python link_to_latest.py docs/"$directory"_tmp/functions-reference "Stan Functions Reference"
+python3 link_to_latest.py docs/"$directory"_tmp/stan-users-guide "Stan User's Guide"
+python3 link_to_latest.py docs/"$directory"_tmp/reference-manual "Stan Reference Manual"
+python3 link_to_latest.py docs/"$directory"_tmp/functions-reference "Stan Functions Reference"
 
 mv docs/"$directory" docs/"$directory"_bak
 mv docs/"$directory"_tmp/ docs/"$directory"