Fix grammar in comment describing LP_DEAD hint safety - Mailing list pgsql-hackers

I noticed a small grammar issue in a comment describing LP_DEAD hint

handling.

The sentence was missing the verb "is" in "whether it is safe to apply

LP_DEAD hints to the page later", which makes the comment grammatically

incorrect.

This patch fixes the comment only and does not change any behavior. 

Regards,

Xiangyu Liang