doc: use .dtso as extension for device tree overlay sources

Moving towards using .dtso for overlay sources, update the
documentation examples to follow that pattern.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/develop/uefi/uefi.rst b/doc/develop/uefi/uefi.rst
index 68f9b33..f8510d3 100644
--- a/doc/develop/uefi/uefi.rst
+++ b/doc/develop/uefi/uefi.rst
@@ -594,10 +594,10 @@
 
 .. code-block:: console
 
-    $ dtc -@ -I dts -O dtb -o version.dtbo version.dts
+    $ dtc -@ -I dts -O dtb -o version.dtbo version.dtso
     $ fdtoverlay -i orig.dtb -o new.dtb -v version.dtbo
 
-where version.dts looks like::
+where version.dtso looks like::
 
     /dts-v1/;
     /plugin/;