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.

I chose to use 'inreplace' rather than a patch as it has a unique
signature in Makefile.in and require less maintenance in future, won't
change anything in the current 2.1.0 distribution and the change is
exclusively for Homebrew.

`brew install fish --HEAD` now builds the new documentation successfully as long as the Doxygen dependency is met.