sudo apt-get install ros-<rosdistro>-rosbridge-suite

The <rosdistro> is a place holder, you are not meant to verbatim copy-paste it into your command, but you should replace it with whatever ROS distribution you are using.

In your case that would be kinetic, so the command would be:

sudo apt-get install ros-kinetic-rosbridge-suite

However: since you installed on a Raspberry Pi: only if you already installed the rest of ROS with apt will this command work. If you used catkin_make and built all packages from source in a workspace you cannot use apt afterwards.