pythongh-125818: Fix incorrect signature of argument `skip_file_prefi…

…xes` in warnings docs (pythonGH-125823)

Change documentation

Doc/library/warnings.rst

@@ -398,7 +398,7 @@ Available Functions
 -------------------
 
 
-.. function:: warn(message, category=None, stacklevel=1, source=None, *, skip_file_prefixes=None)
+.. function:: warn(message, category=None, stacklevel=1, source=None, *, skip_file_prefixes=())
 
    Issue a warning, or maybe ignore it or raise an exception.  The *category*
    argument, if given, must be a :ref:`warning category class <warning-categories>`; it