manual: luarocks: improve tutorial example

The license file in a luarocks package is always inside the subdir, so the
example should reflect this.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

docs/manual/adding-packages-luarocks.txt

@@ -23,7 +23,7 @@ with an example :
 10: LUAFOO_BUILD_OPTS += FOO_INCDIR=$(STAGING_DIR)/usr/include
 11: LUAFOO_BUILD_OPTS += FOO_LIBDIR=$(STAGING_DIR)/usr/lib
 12: LUAFOO_LICENSE = luaFoo license
-13: LUAFOO_LICENSE_FILES = COPYING
+13: LUAFOO_LICENSE_FILES = $(LUAFOO_SUBDIR)/COPYING
 14:
 15: $(eval $(luarocks-package))
 ------------------------