Fix obsolete header-file reference in pg_buffercache docs.

Commit 2d0019049 moved enum ForkNumber from relfilenode.h into relpath.h,
but missed updating this documentation reference.

Alexander Law

doc/src/sgml/pgbuffercache.sgml

@@ -82,7 +82,7 @@
       <entry><type>smallint</type></entry>
       <entry></entry>
       <entry>Fork number within the relation;  see
-      <filename>include/storage/relfilenode.h</></entry>
+      <filename>include/common/relpath.h</></entry>
      </row>
 
      <row>