Updated updater.py

Changed None to ``None`` in the docstring

Signed-off-by: Abhisman Sarkar <abhisman.sarkar@gmail.com>

Author: abs007

tuf/ngclient/updater.py

@@ -183,7 +183,7 @@ def find_cached_target(
 
         Returns:
             Local file path if the file is an up to date target file.
-            None if file is not found or it is not up to date.
+            ``None`` if file is not found or it is not up to date.
         """
 
         if filepath is None: