build: fix README.md reference after rename

The README file is displayed on "make help", but updating the
command has been overlooked during the README -> README.md
rename.

Fix it.

Fixes: d0113711 ("README: port to 21st century")
Signed-off-by: NAdrian Schmutzler <freifunk@adrianschmutzler.de>

include/toplevel.mk

@@ -260,7 +260,7 @@ package/symlinks-clean:
 	./scripts/feeds uninstall -a
 
 help:
-	cat README
+	cat README.md
 
 distclean: cacheclean
 	rm -rf bin build_dir .config* dl feeds key-build* logs package/feeds package/openwrt-packages staging_dir tmp