Clarify the command-line help for the "--config" flag (#919).

NEWS

@@ -1,4 +1,5 @@
 1.8.15.dev0
+ * #919: Clarify the command-line help for the "--config" flag.
  * #918: BREAKING: When databases are configured, don't auto-enable the "one_file_system" option,
    as existing auto-excludes of special files should be sufficient to prevent Borg from hanging on
    them. But if this change causes problems for you, you can always enable "one_file_system"

borgmatic/commands/arguments.py

@@ -300,7 +300,7 @@ def make_parsers():
         '--config',
         dest='config_paths',
         action='append',
-        help=f"Configuration filename or directory, can specify flag multiple times, defaults to: {' '.join(unexpanded_config_paths)}",
+        help=f"Configuration filename or directory, can specify flag multiple times, defaults to: -c {' -c '.join(unexpanded_config_paths)}",
     )
     global_group.add_argument(
         '-n',