[Bug] Update main.py to fix toml-lint (#1202)

(cherry picked from commit e40276c12b)

detection_rules/main.py

@@ -113,8 +113,8 @@ def import_rules(input_file, directory):
 
 
 @root.command('toml-lint')
-@click.option('--rule-file', '-f', type=click.Path(), multiple=True,
-              help='Optionally specify rule files')
+@click.option('--rule-file', '-f', multiple=True, type=click.Path(exists=True),
+              help='Specify one or more rule files.')
 def toml_lint(rule_file):
     """Cleanup files with some simple toml formatting."""
     if rule_file: