Normalize flag style

main.py

@@ -17,7 +17,7 @@ def main():
     )
     parser.add_argument(
         "-p",
-        "--passwd_file",
+        "--passwd-file",
         type=Path,
         help="Path to the passwd file.",
         metavar="PATH",
@@ -25,7 +25,7 @@ def main():
     )
     parser.add_argument(
         "-g",
-        "--group_file",
+        "--group-file",
         type=Path,
         help="Path to the group file.",
         metavar="PATH",
@@ -33,7 +33,7 @@ def main():
     )
     parser.add_argument(
         "-s",
-        "--shadow_file",
+        "--shadow-file",
         type=Path,
         help="Path to the shadow file.",
         metavar="PATH",
@@ -42,7 +42,7 @@ def main():
 
     parser.add_argument(
         "-o",
-        "--output_dir",
+        "--output-dir",
         type=Path,
         help="Path to a directory to store json user records.",
         metavar="OUTPUT_DIR",