ympd: fix license typo

Signed-off-by: Jerzy Grzegorek <jerzy.grzegorek@trzebnica.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/ympd/ympd.mk

@@ -7,7 +7,7 @@
 YMPD_VERSION = a98d760dddff4b0dd595d977c9077f34beff05c5
 YMPD_SITE = $(call github,notandy,ympd,$(YMPD_VERSION))
 YMPD_LICENSE = GPLv2
-YMPD_LICENSE_FILE = LICENSE
+YMPD_LICENSE_FILES = LICENSE
 YMPD_DEPENDENCIES = libmpdclient
 
 define YMPD_MAKE_HOST_TOOL