Documentation/process: fix reST table border error

Fix reST error in Documentation/process/:

Documentation/process/2.Process.rst:131: ERROR: Malformed table.
Bottom/header table border does not match top border.

Fixes: 8962e40c ("docs: update kernel versions and dates in tables")

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Tim Bird <tbird20d@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>