Fix some grammar in documentation of spgist and pgbench

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/ref/pgbench.sgml

@@ -366,7 +366,7 @@ pgbench <optional> <replaceable>options</> </optional> <replaceable>dbname</>
       <term><option>--latency-limit=</option><replaceable>limit</></term>
       <listitem>
        <para>
-        Transaction which last more than <replaceable>limit</> milliseconds
+        Transactions that last more than <replaceable>limit</> milliseconds
         are counted and reported separately, as <firstterm>late</>.
        </para>
        <para>

doc/src/sgml/spgist.sgml

@@ -152,7 +152,7 @@
  <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>