Changes.md: Change OUT_DIR to PRODUCT_OUT

"OUT" means "out/target/proudct/<product>/".
But "OUT_DIR" means "out/".
So we can't use "OUT_DIR" instead of "OUT".

We can use "PRODUCT_OUT" instead of "OUT" which means same folder.

Test: N/A Just document modification
Change-Id: Id08664ba48b226ead7ab6b00d0291259d3a737ba