Fix inconsistent markups in catalogs.sgml

Some fields related to pg_opclass and pg_opfamily were using incorrect
markups, listing them as structname instead of structfield.

Author: Fabien Coelho
Discussion: https://postgr.es/m/alpine.DEB.2.22.394.2006210903560.859381@pseudo

doc/src/sgml/catalogs.sgml

@@ -897,7 +897,7 @@
 
   <para>
    An entry's <structfield>amopmethod</structfield> must match the
-   <structname>opfmethod</structname> of its containing operator family (including
+   <structfield>opfmethod</structfield> of its containing operator family (including
    <structfield>amopmethod</structfield> here is an intentional denormalization of the
    catalog structure for performance reasons).  Also,
    <structfield>amoplefttype</structfield> and <structfield>amoprighttype</structfield> must match
@@ -5064,10 +5064,10 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
 
   <para>
    An operator class's <structfield>opcmethod</structfield> must match the
-   <structname>opfmethod</structname> of its containing operator family.
+   <structfield>opfmethod</structfield> of its containing operator family.
    Also, there must be no more than one <structname>pg_opclass</structname>
-   row having <structname>opcdefault</structname> true for any given combination of
-   <structname>opcmethod</structname> and <structname>opcintype</structname>.
+   row having <structfield>opcdefault</structfield> true for any given combination of
+   <structfield>opcmethod</structfield> and <structfield>opcintype</structfield>.
   </para>
 
  </sect1>