doc: Adjust some markups on pg_waldump page

Author: Peter Smith <smithpb2250@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Discussion: https://postgr.es/m/CAHut+PuuPps9bUPvouU5dH=tOTiF8QBzQox5O7DqXeOFdda79Q@mail.gmail.com

doc/src/sgml/ref/pg_waldump.sgml

@@ -22,8 +22,8 @@ PostgreSQL documentation
  <refsynopsisdiv>
   <cmdsynopsis>
    <command>pg_waldump</command>
-   <arg rep="repeat" choice="opt"><option>option</option></arg>
-   <arg choice="opt"><option>startseg</option><arg choice="opt"><option>endseg</option></arg></arg>
+   <arg rep="repeat" choice="opt"><replaceable>option</replaceable></arg>
+   <arg choice="opt"><replaceable>startseg</replaceable><arg choice="opt"><replaceable>endseg</replaceable></arg></arg>
   </cmdsynopsis>
  </refsynopsisdiv>