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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/w1/slaves/w1_ds2438.rst

···
 to userspace, otherwise an I/O error is returned.
 
 "offset"
--------
+--------
 This file controls the 2-byte Offset Register of the chip.
 Writing a 2-byte value will change the Offset Register, which changes the
 current measurement done by the chip. Changing this register to the two's complement