tiny fix: Adds match! as a valid keyword

[MMagueta]

I noticed match! was not being understood as a valid keyword, so I just added it.

My steps:

• nix-shell -p cask emacs python38 git which gnumake --pure
• make
• make run-fsharp-mode
• Testes highlighting on the scratch buffer after activating the mode

Note: Thanks for the great work on this repository so far

@juergenhoetzel

[juergenhoetzel]

Thanks you! :+1: