Makefile: update warning about CONFIG_OF_EMBED

Update the diagnostic message with revised location of document, which
changed in 3e9fddfc4f1 ("doc: Move devicetree control doc to rST")

Signed-off-by: Ralph Siemsen <ralph.siemsen@linaro.org>
diff --git a/Makefile b/Makefile
index d394340..e3ce287 100644
--- a/Makefile
+++ b/Makefile
@@ -1120,7 +1120,7 @@
 	@echo >&2 "CONFIG_OF_EMBED is enabled. This option should only"
 	@echo >&2 "be used for debugging purposes. Please use"
 	@echo >&2 "CONFIG_OF_SEPARATE for boards in mainline."
-	@echo >&2 "See doc/README.fdt-control for more info."
+	@echo >&2 "See doc/develop/devicetree/control.rst for more info."
 	@echo >&2 "===================================================="
 endif
 ifneq ($(CONFIG_SPL_FIT_GENERATOR),)