docs: minor typo fix for "lower(anymultirange)"

Hi

Here:

     https://www.postgresql.org/docs/current/functions-range.html#MULTIRANGE-FUNCTIONS-TABLE

the description for "lower(anymultirange)":

> (NULL if the multirange is empty has no lower bound).

is missing "or" and should be:

> (NULL if the multirange is empty or has no lower bound).

Seems to have been omitted with 7539a1b2 (REL_16_STABLE + master).

Very minor issue, but I have seen it and can no longer unsee it.

Regards

Ian Barwick