Documentation: ARM: fix reference to DT format documentation

The booting-without-of.rst file is no longer there. Link to
devicetree.org instead.

Fixes: 44184828 ("dt: Remove booting-without-of.rst")
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Cc: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/7f07e544d9fc584242d496c2f54f9303d8de0724.1611558630.git.baruch@tkos.co.ilSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/arm/booting.rst

@@ -128,7 +128,7 @@ it.  The recommended placement is in the first 16KiB of RAM.
 
 The boot loader must load a device tree image (dtb) into system ram
 at a 64bit aligned address and initialize it with the boot data.  The
-dtb format is documented in Documentation/devicetree/booting-without-of.rst.
+dtb format is documented at https://www.devicetree.org/specifications/.
 The kernel will look for the dtb magic value of 0xd00dfeed at the dtb
 physical address to determine if a dtb has been passed instead of a
 tagged list.