Fixes fstring format in os.walk example

REF: Issue 336

Author: LairdStreak

docs/modules/os-module.md

@@ -217,9 +217,9 @@ You can get a relative path from a starting path to another path.
 >>> for folder_name, subfolders, filenames in os.walk('C:\\delicious'):
 ...     print(f'The current folder is {folder_name}')
 ...     for subfolder in subfolders:
-...         print('SUBFOLDER OF {folder_name}: {subfolder}')
+...         print(f'SUBFOLDER OF {folder_name}: {subfolder}')
 ...     for filename in filenames:
-...         print('FILE INSIDE {folder_name}: filename{filename}')
+...         print(f'FILE INSIDE {folder_name}: filename{filename}')
 ...     print('')
 ...
 # The current folder is C:\delicious