pageinspect: use index_close() for GiST index relation

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