docs: dt: writing-schema: Remove spurious indentation

'allOf' and 'properties' have a leading space which causes them to be
indented in the doc output.

Cc: Frank Rowand <frowand.list@gmail.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/20210325164713.1296407-3-robh@kernel.org

Documentation/devicetree/writing-schema.rst

@@ -46,12 +46,12 @@ select
   schema. By default without 'select', nodes are matched against their possible
   compatible string values or node name. Most bindings should not need select.
 
- allOf
+allOf
   Optional. A list of other schemas to include. This is used to
   include other schemas the binding conforms to. This may be schemas for a
   particular class of devices such as I2C or SPI controllers.
 
- properties
+properties
   A set of sub-schema defining all the DT properties for the
   binding. The exact schema syntax depends on whether properties are known,
   common properties (e.g. 'interrupts') or are binding/vendor specific properties.