mentioned default color in docstring

Author: immaxchen

pandas/io/formats/style.py

@@ -1012,7 +1012,7 @@ def highlight_null(self, null_color: str = "red", subset=None) -> "Styler":
 
         Parameters
         ----------
-        null_color : str
+        null_color : str, default 'red'
         subset : IndexSlice, default None
             A valid slice for ``data`` to limit the style application to.