Doc: Fix incorrect mention to connection_object in CONNECT command of ECPG

This fixes an inconsistency with this parameter name not listed in the
command synopsis, and connection_name is the parameter name more
commonly used in the docs for ECPG commands.

Reported-by: Yusuke Egashita
Discussion: https://postgr.es/m/156870956796.1259.11456186889345212399@wrigleys.postgresql.org
Backpatch-through: 9.4

doc/src/sgml/ecpg.sgml

@@ -6512,7 +6512,7 @@ DATABASE <replaceable>connection_target</replaceable>
      </varlistentry>
 
      <varlistentry>
-      <term><replaceable class="parameter">connection_object</replaceable></term>
+      <term><replaceable class="parameter">connection_name</replaceable></term>
       <listitem>
        <para>
         An optional identifier for the connection, so that it can be