nvdimm/blk: Fix title level

nvdimm/blk: Fix title level

make htmldocs fails with
Sphinx parallel build error:
docutils.utils.SystemMessage: ...nvdimm.rst:146:
  (SEVERE/4) Title level inconsistent:

PMEM-REGIONs, Atomic Sectors, and DAX
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The PMEM vs BLK section was removed without changing
the PMEM-REGIONS... title line.  Replace '^' with '_'.

Fixes: f8669f1d6a86 ("nvdimm/blk: Delete the block-aperture window driver")
Signed-off-by: Tom Rix <trix@redhat.com>
Link: https://lore.kernel.org/r/20220321220408.2381974-1-trix@redhat.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>