Trivial markup improvement.

doc/src/sgml/func.sgml

 <!--
-$PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.250 2005/05/23 01:50:01 neilc Exp $
+$PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.251 2005/06/06 16:29:01 tgl Exp $
 PostgreSQL documentation
 -->
 
@@ -2725,7 +2725,7 @@ cast(-44 as bit(12))           <lineannotation>111111010100</lineannotation>
 
     <para>
      The <function>substring</> function with three parameters,
-     <function>substring(<parameter>string</parameter> from
+     <function>substring(<replaceable>string</replaceable> from
      <replaceable>pattern</replaceable> for
      <replaceable>escape-character</replaceable>)</function>, provides
      extraction of a substring that matches an SQL
@@ -2837,7 +2837,7 @@ substring('foobar' from '#"o_b#"%' for '#')    <lineannotation>NULL</lineannotat
 
     <para>
      The <function>substring</> function with two parameters,
-     <function>substring(<parameter>string</parameter> from
+     <function>substring(<replaceable>string</replaceable> from
      <replaceable>pattern</replaceable>)</function>, provides extraction of a substring
      that matches a POSIX regular expression pattern.  It returns null if
      there is no match, otherwise the portion of the text that matched the