Point recipe HEAD to fishman/ctags

Ctags 5.8 dates from July 2009, which would be fine if no programming language had evolved or been created since. The most active and consistently updated fork appears to be [fishman/ctags](https://github.com/fishman/ctags), but it is neither official nor properly versioned. Pointing only `--HEAD` to that fork will allow advanced users to get a current, actively developed version while keeping the known 5.8 release as the regular install.