Error on `Building Arduino` wiki page for MacOS brew command

[ephsiung]

The Building Arduino wiki page has an incorrect command line in the Mac OSX section. The command line:

• brew cask install adoptopenjdk8

should actually be:

• brew install cask adoptopenjdk8

[ubidefeo]

@per1234 @ephsiung

the correct command is actually

#> brew install --cask adoptopenjdk8