Fix rviz exec dependency in documentation.

[ValerioMagnago]

Easy to confuse between ros 1 and ros 2 but the correct package for ros 2 is rviz2

[SteveMacenski]

No it is not https://github.com/ros/rosdistro/blob/master/rolling/distribution.yaml#L5477 We need the full metapackage. You'd be correct for the base rviz but we want to get the default plugins and renders and whatnot if they're not already installed. They're used in later tutorials.