Put listdir default argument into declaration.

Doc/library/os.rst

@@ -1049,10 +1049,10 @@ Files and Directories
    Availability: Unix.
 
 
-.. function:: listdir([path])
+.. function:: listdir(path='.')
 
    Return a list containing the names of the entries in the directory given by
-   *path* (default: '.').  The list is in arbitrary order.  It does not include the special
+   *path* (default: ``'.'``).  The list is in arbitrary order.  It does not include the special
    entries ``'.'`` and ``'..'`` even if they are present in the directory.
 
    This function can be called with a bytes or string argument, and returns