[doc] fix tree

docs/colmap_arborescence.md

@@ -6,6 +6,7 @@ If your project is called `my_project` and is stored in `/path/to/my_project`
 then if you were to look at the `tree` inside `/path/to/my_projet` after COLMAP
 has been run you should see something like.
 
+```
 +── images
 │   +── image1.jpg
 │   +── image2.jpg
@@ -26,6 +27,7 @@ has been run you should see something like.
 │   │   +── meshed-delaunay.ply
 │   +── ...
 +── database.db
+```
 
 If you want to learn more about the arborescence of a COLMAP project please
 refeer to the official COLMAP documentation at htps://colmap.github.io