Document default parameter of .seek() in the signature. (GH-14691)

(cherry picked from commit 2a3d4d9c53dd4831c3ecf56bc7c4a289c33030d6)

Co-authored-by: Benjamin Peterson <benjamin@python.org>
diff --git a/Doc/library/io.rst b/Doc/library/io.rst
index fce9a74..70e0115 100644
--- a/Doc/library/io.rst
+++ b/Doc/library/io.rst
@@ -327,7 +327,7 @@
       Note that it's already possible to iterate on file objects using ``for
       line in file: ...`` without calling ``file.readlines()``.
 
-   .. method:: seek(offset[, whence])
+   .. method:: seek(offset, whence=SEEK_SET)
 
       Change the stream position to the given byte *offset*.  *offset* is
       interpreted relative to the position indicated by *whence*.  The default
@@ -831,7 +831,7 @@
 
       If *size* is specified, at most *size* characters will be read.
 
-   .. method:: seek(offset[, whence])
+   .. method:: seek(offset, whence=SEEK_SET)
 
       Change the stream position to the given *offset*.  Behaviour depends on
       the *whence* parameter.  The default value for *whence* is