fix the example so that all leaves are regular files

also fix typo

modules/home-assignments/dirtree.md

@@ -1,16 +1,16 @@
 ## Create and traverse path tree
 
 In memory, build a tree of paths, where each node is a path element. The tree
-starts with root '/', the non-leaf nodes are directories, the leaves are files.
-The three can look e.g. like this:
+starts with root `/` (assuming Unix-like system), the non-leaf nodes are directories, the leaves are regular files.
+The tree can look e.g. like this for paths `/foo/a.c`, `/foo/b.c`, `/f.txt`, `/bar/c.c`:
 
 ```
 	 "/"
        /  |   \
       /   |     \
   "foo/" "f.txt"  "bar/"
-  /   \
-"a.c" "b.c"
+  /   \            |
+"a.c" "b.c"       "c.c"
 ```
 
 Populate the tree with given paths and print the leaves (no particular order),