fish: update for new documentation build process.

Part of an updated documentation build process, the lexicon_filter
script's 'sed' shebang is set during make using 'which'. In Homebrew's
'superenv' this will be incorrect as the superenv stub isn't
shebangable, so it needs to be rewritten.

Closes #32315.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>