Docs: fix misleading example.
Docs: fix misleading example.

Commit 8457d0beca731bf0 introduced an example which, while not incorrect,
failed to exhibit the behavior it meant to describe, as a result of omitting
an E'' prefix that needed to be there.  Noticed and fixed by Peter Geoghegan.

I (tgl) failed to resist the temptation to wordsmith nearby text a bit
while at it.

zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl