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.
It could... I authored the new docs and the dynamic shebang is to account for the differences in BSD/GNU sed location across distros. So either it's a case of adding singular  Homebrew `superenv` detection to the make file, dropping back to `env :std` or handling the brew exception in the formula. I've been pondering on that one since this morning and came to the decision that the formula was the best place for it, just from a KISS perspective.