Fix document bug regarding read only transactions.

It was explained that read only transactions (not in standby) allow to
update sequences. This had been wrong since the commit:
05d8a56

Discussion: https://www.postgresql.org/message-id/20170614.110826.425627939780392324.t-ishii%40sraoss.co.jp

Author: tatsuo-ishii

doc/src/sgml/high-availability.sgml

@@ -1783,7 +1783,7 @@ if (!triggered)
 
    <para>
     In normal operation, <quote>read-only</> transactions are allowed to
-    update sequences and to use <command>LISTEN</>, <command>UNLISTEN</>, and
+    use <command>LISTEN</>, <command>UNLISTEN</>, and
     <command>NOTIFY</>, so Hot Standby sessions operate under slightly tighter
     restrictions than ordinary read-only sessions.  It is possible that some
     of these restrictions might be loosened in a future release.