doc: Make selectivity example match wording

Hi,

Reposting this to its own thread.

https://www.postgresql.org/message-id/flat/CAKFQuwby1aMsJDMeibaBaohgoaZhivAo4WcqHC1%3D9-GDZ3TSng%40mail.gmail.com

doc: make unique non-null join selectivity example match the prose

The description of the computation for the unique, non-null,
join selectivity describes a division by the maximum of two values,
while the example shows a multiplication by their reciprocal. While
equivalent the max phrasing is easier to understand; which seems
more important here than precisely adhering to the formula used
in the code (for which either variant is still an approximation).

While both num_distinct and num_rows are equal for a unique column
both the concept and formula use row count (10,000) and the
field num_distinct has already been set to mean the specific value
present in the pg_stats table (i.e, -1), so use num_rows here.

David J.