doc: fix for commit 09f7d36ba1 in changing "_" to "-".

I thought underscores wouldn't even work in "id"s, so I never checked to
see if anything referenced it, but it seems it does work, so adjust the
calling site for the dash syntax.

doc/src/sgml/ref/alter_database.sgml

@@ -83,7 +83,7 @@ ALTER DATABASE <replaceable class="parameter">name</replaceable> RESET ALL
    must be empty for this database, and no one can be connected to
    the database.  Tables and indexes in non-default tablespaces are
    unaffected.  The method used to copy files to the new tablespace
-   is affected by the <xref linkend="guc_file_copy_method"/> setting.
+   is affected by the <xref linkend="guc-file-copy-method"/> setting.
   </para>
 
   <para>

doc/src/sgml/ref/create_database.sgml

@@ -140,7 +140,7 @@ CREATE DATABASE <replaceable class="parameter">name</replaceable>
         after the creation of the new database. In some situations, this may
         have a noticeable negative impact on overall system performance. The
         <literal>FILE_COPY</literal> strategy is affected by the <xref
-        linkend="guc_file_copy_method"/> setting.
+        linkend="guc-file-copy-method"/> setting.
        </para>
       </listitem>
      </varlistentry>