kbuild: fix comment about dst of headers_{install, check}_all

Commit 61562f98 ("uapi: export all arch specifics directories")
changed the dst from asm-<arch> to arch-<arch> for headers_install_all
or headers_check_all.  Update the comment.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -1133,7 +1133,7 @@ firmware_install:
 #Default location for installed headers
 export INSTALL_HDR_PATH = $(objtree)/usr
 
-# If we do an all arch process set dst to asm-$(hdr-arch)
+# If we do an all arch process set dst to include/arch-$(hdr-arch)
 hdr-dst = $(if $(KBUILD_HEADERS), dst=include/arch-$(hdr-arch), dst=include)
 
 PHONY += archheaders