dt-bindings: writing-schema: Install from PyPI repository

Installing dtschema from github.com/devicetree-org is not needed anymore
because dtschema is now part of regular PyPI repository.  In certain
cases it might cause some troubles as it brings latest master version,
not the stable release:

    $ pip3 show dtschema
    Version: 2020.8.2.dev4+g341f3e3
    $ make dt_binding_check
    dtschema minimum version is v2020.8.1
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220223073547.8746-1-krzysztof.kozlowski@canonical.com

Documentation/devicetree/bindings/writing-schema.rst

@@ -118,7 +118,7 @@ The DT schema project must be installed in order to validate the DT schema
 binding documents and validate DTS files using the DT schema. The DT schema
 project can be installed with pip::
 
-    pip3 install git+https://github.com/devicetree-org/dt-schema.git@master
+    pip3 install dtschema
 
 Several executables (dt-doc-validate, dt-mk-schema, dt-validate) will be
 installed. Ensure they are in your PATH (~/.local/bin by default).