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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dan Williams <[e-mail]>

Documentation/driver-api/nvdimm/nvdimm.rst

···
 	    Supporting Documents
 	    Git Trees
 	LIBNVDIMM PMEM
-	        PMEM-REGIONs, Atomic Sectors, and DAX
+	    PMEM-REGIONs, Atomic Sectors, and DAX
 	Example NVDIMM Platform
 	LIBNVDIMM Kernel Device Model and LIBNDCTL Userspace API
 	    LIBNDCTL: Context
···
 label-less or "legacy".
 
 PMEM-REGIONs, Atomic Sectors, and DAX
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+-------------------------------------
 
 For the cases where an application or filesystem still needs atomic sector
 update guarantees it can register a BTT on a PMEM device or partition.  See