[openwrt/openwrt] build: fix README.md reference after rename

[LEDE Commits]

adrian pushed a commit to openwrt/openwrt.git, branch master:
https://git.openwrt.org/751486b31fd9b6fb489bc419d27a13460ade23e1

commit 751486b31fd9b6fb489bc419d27a13460ade23e1
Author: Adrian Schmutzler <freifunk at adrianschmutzler.de>
AuthorDate: Wed Aug 12 13:42:33 2020 +0200

    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: d0113711a31f ("README: port to 21st century")
    
    Signed-off-by: Adrian Schmutzler <freifunk at adrianschmutzler.de>
---
 include/toplevel.mk | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/toplevel.mk b/include/toplevel.mk
index 0a64afe153..c7e71ad571 100644
--- a/include/toplevel.mk
+++ b/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