Fix typo in JDK install instructions

[Kafkaese]

Due to the missing dash the code will yield the following error:

xcode-select: error: invalid argument '-install'

since the option needs two dashes and has no shorthand flag.

Note: I did not only add an extra dash, but replaced the original single dash with two dashes. This is because the original dash was a different dash from the one generated by my keyboard. (See this issue) So anyone who copy-pastes the instructions would have run into the issue mentioned in the post.

Now I have no idea wether this will lead to issues for other users that use a different keyboard. My hope is that the encoding depends on the OS and the issues originated from a non-mac users typing (or copy-pasting) the instructions into the readme. It could also be terminal-specific. I don't know.