docs: fix mise install docs for zsh

Closes #18

Switches to the `zsh` builtin command `whence` to resolve the location
of the mise binary:

https://zsh.sourceforge.io/Doc/Release/Shell-Builtin-Commands.html

docs/developer-setup.md

@@ -150,7 +150,7 @@ mise settings set legacy_version_file false
 # bash
 echo 'eval "$('$(which mise)' activate bash)"' >> ~/.bashrc
 # zsh
-echo 'eval "$('$(which mise)' activate zsh)"' >> ~/.zshrc
+echo 'eval "$('$(whence -cp mise)' activate zsh)"' >> ~/.zshrc
 # fish
 echo 'mise activate fish | source' >> ~/.config/fish/config.fish
 ```