Switch more function arguments docs to new-style.
diff --git a/Doc/library/os.path.rst b/Doc/library/os.path.rst
index 6d177f8..8f82963 100644
--- a/Doc/library/os.path.rst
+++ b/Doc/library/os.path.rst
@@ -218,7 +218,7 @@
    links encountered in the path (if they are supported by the operating system).
 
 
-.. function:: relpath(path[, start])
+.. function:: relpath(path, start=None)
 
    Return a relative filepath to *path* either from the current directory or from
    an optional *start* point.