pageinspect: use index_close() for GiST index relation

gist_page_items() opens its target relation with index_open(), but
closed it using relation_close() instead of index_close().  This was
harmless because index_close() and relation_close() do the exact same
work, still inconsistent with the rest of the code tree as routines
opening and closing a relation based on a relkind are expected to match,
at least in name.

Author: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/CAEoWx2=bL41WWcD-4Fxx-buS2Y2G5=9PjkxZbHeFMR6Uy2WNvw@mail.gmail.com

contrib/pageinspect/gistfuncs.c

@@ -361,7 +361,7 @@ gist_page_items(PG_FUNCTION_ARGS)
 		tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc, values, nulls);
 	}
 
-	relation_close(indexRel, AccessShareLock);
+	index_close(indexRel, AccessShareLock);
 
 	return (Datum) 0;
 }