Fix debian package installation instructions

[micah]

The correct command to run is dpkg -i on a debian package file. The instructions said to use apt install which does not work to install from a file.

[ReneNyffenegger]

I was able to install dive with exactly the instructions given in the README.md for Debian.

[edwardbaidoo]

The correct command to run is dpkg -i on a debian package file. The instructions said to use apt install which does not work to install from a file.

I believe apt install was used here since it would proceed to install missing dependencies(If any) as it installs the package.