Update wiring info

[rvangeijtenbeek]

Updated the description of 'slice wires' after 3.0 release, as this action uses the Alt key + left-mouse button now. Also made the markup for keyboard keys consistent. Fixes https://github.com/node-red/node-red.github.io/issues/296

[linux-foundation-easycla[bot]]

The committers listed above are authorized under a signed CLA.

• :white_check_mark: login: rvangeijtenbeek (a01ca28ba3a5592654d44eed6f3c86f167bf3974)