Fix typo in gnu-which formula name

Fix the typo in the formula name of `gnu-which` (1) that is currently
stored as `gnu-witch`.
Sometimes auto-complete is evil.

References:
  (1) https://formulae.brew.sh/formula/gnu-which

Resolves GH-188

snowflakes/homebrew/Brewfile

@@ -64,7 +64,7 @@ brew "git-crypt"
 brew "git-sizer"
 brew "gnu-sed", args: ["with-default-names"]
 brew "gnu-tar", args: ["with-default-names"]
-brew "gnu-witch", args: ["with-default-names"]
+brew "gnu-which", args: ["with-default-names"]
 brew "hostess"
 brew "htop", args: ["with-ncurses"]
 brew "httpie"