doc: fix typo "vertexes" -> "vertices"

The "vertexes" spelling is also valid, but we consistently use
"vertices" elsewhere.

Author: Dagfinn Ilmari Mannsåker
Reviewed-by: Shubham Khanna
Discussion: https://postgr.es/m/87le9fmi01.fsf@wibble.ilmari.org

Author: michaelpq

doc/src/sgml/datatype.sgml

@@ -3590,7 +3590,7 @@ SELECT person.name, holidays.num_weeks FROM person, holidays
     </indexterm>
 
     <para>
-     Polygons are represented by lists of points (the vertexes of the
+     Polygons are represented by lists of points (the vertices of the
      polygon). Polygons are very similar to closed paths; the essential
      difference is that a polygon is considered to include the area
      within it, while a path is not.