markdownlint: enable rule MD021

MD021 - Multiple spaces inside hashes on closed atx style header

Signed-off-by: Maciej Wawryk <maciejx.wawryk@intel.com>
Change-Id: I27a16ab517ac098b4462187f89cda44fb259da44
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/9039


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Community-CI: Broadcom CI <spdk-ci.pdl@broadcom.com>
Reviewed-by: Paul Luse <paul.e.luse@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Aleksey Marchuk <alexeymar@mellanox.com>