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.