Fix ABAC alt text

[serprex]

As is the text renders as part of the prose, appearing to be missing a period

[serprex]

aside: "An items table to represent subjects and objects" is confusing when the diagram calls the table Objects. The word "items" only appears 3 times, twice in describing the tables (where afaict it would be clearer as objects) & in the SQL query where it appears items is instead the somewhat unlabeled array of triplets under AccessPolicy (is that an array, or is the body only a single triplet which we are calling an array? "array" only appears once in text, in SQL query)