doc: add missing word "the"

Reported-by: doughale@gmail.com

Discussion: https://postgr.es/m/170993253510.640.5664117187431542912@wrigleys.postgresql.org

Backpatch-through: 12

Author: bmomjian

doc/src/sgml/high-availability.sgml

@@ -1290,7 +1290,7 @@ synchronous_standby_names = 'ANY 2 (s1, s2, s3)'
     <varname>synchronous_commit</varname> is set to <literal>on</literal>,
     <literal>remote_apply</literal> or <literal>remote_write</literal> will wait for
     responses from. Such transaction commits may never be completed
-    if any one of synchronous standbys should crash.
+    if any one of the synchronous standbys should crash.
    </para>
 
    <para>