Fix a possible typo in rewriteheap.c code comments

Hi,

It looks like there's a typo, attaching a tiny patch to fix it.

  *
  * When doing logical decoding - which relies on using cmin/cmax of catalog
  * tuples, via xl_heap_new_cid records - heap rewrites have to log enough
- * information to allow the decoding backend to updates its internal mapping
+ * information to allow the decoding backend to update its internal mapping

Regards,
Bharath Rupireddy.