Use brew formulae instead of the deprecated brew search with NoArgs

[densketch]

With Homebrew update, brew search with no arguments is deprecated.
And then they recommending to use brew formulae instead of that.

[laughedelic]

Hi @densketch! Thanks for the fix, I will merge it, but beware that this repository is not maintained anymore because the completions have been merged upstream: #9. So you should have brew completions out of the box without the need to install any fish plugins. I will add a note to the readme here.

If you find time I recommend sending this pull request to the homebrew repo to fix it for all users. Never mind, this has been fixed already.