stubby: fix config file definition

The config file /etc/stubby/stubby.yml is not registered properly and any
local changes are being overwritten on upgrade or reinstall.

Signed-off-by: Tony Ambardar <itugrok@yahoo.com>

net/stubby/Makefile

@@ -46,7 +46,7 @@ define Package/stubby/description
 endef
 
 define Package/stubby/conffiles
-	/etc/stubby/stubby.yml
+/etc/stubby/stubby.yml
 endef
 
 define Package/stubby/install