docs: dt: minor adjustments at writing-schema.rst

There are two literal blocks that aren't mark as such. Mark them,
in order to make the document to produce a better html output.

While here, also add a SPDX header to it.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/14fc680fd6596b277f94bb5a240cc9dfc41d59bf.1592203542.git.mchehab+huawei@kernel.orgSigned-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/writing-schema.rst

-:orphan:
+.. SPDX-License-Identifier: GPL-2.0
 
 Writing DeviceTree Bindings in json-schema
 ==========================================
@@ -124,9 +124,12 @@ dtc must also be built with YAML output support enabled. This requires that
 libyaml and its headers be installed on the host system. For some distributions
 that involves installing the development package, such as:
 
-Debian:
+Debian::
+
   apt-get install libyaml-dev
-Fedora:
+
+Fedora::
+
   dnf -y install libyaml-devel
 
 Running checks