[PATCH] fix doc example of bit-reversed MAC address

Pretty trivial since this is documenting something that Postgres
*doesn't* do, but it incorrectly reversed only the bits of each
nibble, not the whole byte. See e.g.
https://www.ibm.com/docs/en/csfdcd/7.1?topic=ls-bit-ordering-in-mac-addresses
for a handy table.