Adding doctest for md_prefix function in build_directory_md.py (#12874)

* Adding doctests for md_prefix function in build_directory_md.py

* Update build_directory_md.py

* Update build_directory_md.py

* Update build_directory_md.py

---------

Co-authored-by: Maxim Smolskiy <mithridatus@mail.ru>

Author: diya94

scripts/build_directory_md.py

@@ -18,8 +18,20 @@ def good_file_paths(top_dir: str = ".") -> Iterator[str]:
                 yield os.path.join(dir_path, filename).lstrip("./")
 
 
-def md_prefix(i):
-    return f"{i * '  '}*" if i else "\n##"
+def md_prefix(indent: int) -> str:
+    """
+    Markdown prefix based on indent for bullet points
+
+    >>> md_prefix(0)
+    '\\n##'
+    >>> md_prefix(1)
+    '  *'
+    >>> md_prefix(2)
+    '    *'
+    >>> md_prefix(3)
+    '      *'
+    """
+    return f"{indent * '  '}*" if indent else "\n##"
 
 
 def print_path(old_path: str, new_path: str) -> str: