Installation instructions for debian launcher are incorrect

The instruction for command line installation of deb packages on the [download page|https://www.minecraft.net/en-us/download/] are incorrect; the instructions say to use `apt-get install Minecraft.deb`. This will not work as `apt-get` takes a package name argument referring to a package in a repo index rather than a local filename argument. The correct instruction would be `dpkg -i Minecraft.deb` (variously with a `sudo` prefix).