Tips-N'-Tricks: fix symlink for zsh completion

From `Library/Contributions/brew_zsh_completion.zsh`:

````bash
````

Then it should give symlinked file name of `_brew` instead of being just `brew_zsh_completion.zsh`

Closes Homebrew/homebrew#48897.

Signed-off-by: Dominyk Tiller <dominyktiller@gmail.com>