Fix documented default of auto_level

pymdownx/blocks/caption.py

@@ -278,7 +278,7 @@ def __init__(self, *args, **kwargs):
             ],
             "auto_level": [
                 0,
-                "Depth of children to add prefixes to - Default: 1"
+                "Depth of children to add prefixes to - Default: 0"
             ],
             "auto": [
                 False,