w1: fix build warning in w1_ds2438.rst

w1: fix build warning in w1_ds2438.rst

commit c999fbbdcf77 ("w1: ds2438: support for writing to offset
register") added more documentation, but had a one-off line for the
header of a section which caused the build warning:

	Documentation/w1/slaves/w1_ds2438.rst:56: WARNING: Title underline too short.

Resolve this by fixing the underline to be long enough.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Link: https://lore.kernel.org/r/YKthRzCGan9WEcmP@kroah.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>