doc: fit: Format image tree source example

doc: fit: Format image tree source example

The example in kernel_fdt.rst is inconsistently indented, making it
difficult to read.

Indent the example with the same standard as the other examples:
Four spaces for the ReST code block and for every nesting level.

Signed-off-by: J. Neuschäfer <j.ne@posteo.net>