Please re-type the quotes of the code. As you can see, the quotes are crooked, and the first one is upside down. This is usually done for pretty formatting in articles and the like, but is invalid PHP. If the WP_ALLOW_MULTISITE turns green, like you can see with WP_DEBUG and ABSPATH, you should be OK.
Official sources can be wrong too. But the quotes seem correct on the official documentation, so it seems that the quotes got replaced somewhere else.