Fix some grammar in documentation of spgist

Discussion: https://postgr.es/m/92961161-9b49-e42f-0a72-d5d47e0ed4de@postgrespro.ru
Author: Liudmila Mantrova
Reviewed-by: Jonathan Katz, Tom Lane, Michael Paquier
Backpatch-through: 9.4

doc/src/sgml/spgist.sgml

@@ -134,7 +134,7 @@
  <para>
   Of the two operator classes for type <type>point</>,
   <literal>quad_point_ops</> is the default.  <literal>kd_point_ops</>
-  supports the same operators but uses a different index data structure which
+  supports the same operators but uses a different index data structure that
   may offer better performance in some applications.
  </para>