brew installation, update documentation

[evanbiederstedt]

Hi there

Low priority request:

https://bedtools.readthedocs.io/en/latest/content/installation.html

brew tap homebrew/science
brew install bedtools

It looks like brew tap homebrew/science is no longer functional:

$ brew tap homebrew/science
Error: homebrew/science was deprecated. This tap is now empty as all its formulae were migrated.```

[zichenzhang04]

Same issue here!