Add -r option in the custom rule script documentation

Note:
--recursive, -r       Recursively scan subfolders in a directory

Change-Id: I0c1255d250c36a9e6e6f5caeeb1d786f4c723655
Signed-off-by: George Keishing <gkeishin@in.ibm.com>
diff --git a/robot_custom_rules.py b/robot_custom_rules.py
index 39eecdf..1c917f2 100644
--- a/robot_custom_rules.py
+++ b/robot_custom_rules.py
@@ -1,6 +1,6 @@
 #!/usr/bin/env python
 # Custom rules file for robotframework-lint.
-# Example usage: python -m rflint -A robot_standards -R robot_custom_rules.py .
+# Example usage: python -m rflint -rA robot_standards -R robot_custom_rules.py .
 import re
 from rflint.common import SuiteRule, ERROR