Support for multiple PRODUCT_SDK_ADDON_DOC_MODULES

Bug: 5429077
Change-Id: I8d962c803496479ba5b5e7316538aced65ee4df1
diff --git a/core/product.mk b/core/product.mk
index 26f365d..f48d2be 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -81,7 +81,7 @@
     PRODUCT_SDK_ADDON_NAME \
     PRODUCT_SDK_ADDON_COPY_FILES \
     PRODUCT_SDK_ADDON_COPY_MODULES \
-    PRODUCT_SDK_ADDON_DOC_MODULE \
+    PRODUCT_SDK_ADDON_DOC_MODULES \
     PRODUCT_DEFAULT_WIFI_CHANNELS \
     PRODUCT_DEFAULT_DEV_CERTIFICATE \