doc: networking: shorten the main title in offloads documents

The titles do not look very nice in the table of contents generated by
Sphinx.

I also think it is obvious that the documents are describing offloads
in the Linux Networking Stack.
Signed-off-by: Otto Sabart <ottosabart@seberm.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/networking/checksum-offloads.rst

 .. SPDX-License-Identifier: GPL-2.0
 
-===============================================
-Checksum Offloads in the Linux Networking Stack
-===============================================
+=================
+Checksum Offloads
+=================
 
 
 Introduction

Documentation/networking/segmentation-offloads.rst

 .. SPDX-License-Identifier: GPL-2.0
 
-===================================================
-Segmentation Offloads in the Linux Networking Stack
-===================================================
+=====================
+Segmentation Offloads
+=====================
 
 
 Introduction