fix a typo for single PRODUCT_PRODUCT_VNDK_VERSION

Bug: 144534640
Test: build

Signed-off-by: GeQi <geqi@xiaomi.com>
Change-Id: I5bf47abd70e84e89930846c25b2592dac1100082
diff --git a/core/product.mk b/core/product.mk
index 324010c..624501e 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -323,7 +323,7 @@
 
 # VNDK version of product partition. It can be 'current' if the product
 # partitions uses PLATFORM_VNDK_VERSION.
-_product_single_value_var += PRODUCT_PRODUCT_VNDK_VERSION
+_product_single_value_vars += PRODUCT_PRODUCT_VNDK_VERSION
 
 # Whether the list of allowed of actionable compatible properties should be disabled or not
 _product_single_value_vars += PRODUCT_ACTIONABLE_COMPATIBLE_PROPERTY_DISABLE