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: f8669f1 ("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>

Author: Tom Rix

Documentation/driver-api/nvdimm/nvdimm.rst

@@ -15,7 +15,7 @@ Version 13
 	    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
@@ -144,7 +144,7 @@ can be immediately attached to nd_pmem. This latter mode is called
 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