markdownlint: enable rule MD020

MD020 - No space inside hashes on closed atx style header

Signed-off-by: Maciej Wawryk <maciejx.wawryk@intel.com>
Change-Id: Ic81a24e3e23d77cf2436ebda17c4ac09c4f4e0ea
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/9038


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>