Clarify shortcut usage in Carla simulation mode (#1724)

The keys to control openpilot have to be pressed with focus on the
terminal running the Carla bridge.
Also improve table formatting.

tools/sim/README.md

@@ -10,31 +10,31 @@ git clone https://github.com/commaai/openpilot.git
 # Add export PYTHONPATH=$HOME/openpilot to your bashrc
 # Have a working tensorflow+keras in python3.7.3 (with [packages] in openpilot/Pipfile)
 ```
-## Install (in tab 1)
+## Install (in terminal 1)
 ```
 cd ~/openpilot/tools/sim
 ./start_carla.sh  # install CARLA 0.9.7 and start the server
 ```
-## openpilot (in tab 2)
+## openpilot (in terminal 2)
 ```
 cd ~/openpilot/selfdrive/
 PASSIVE=0 NOBOARD=1 ./manager.py
 ```
-## bridge (in tab 3)
+## bridge (in terminal 3)
 ```
 # links carla to openpilot, will "start the car" according to manager
 cd ~/openpilot/tools/sim
 ./bridge.py
 ```
 ## Controls
-Now you can control the simulator with the keys:
-
-1: Cruise up 5 mph
-
-2: Cruise down 5 mph
-
-3: Cruise cancel
-
-q: Exit all
+Now put the focus on the terminal running bridge.py and you can control
+openpilot driving in the simulation with the following keys
+
+|  key  |   functionality   |
+| :---: | :---------------: |
+|   1   |  Cruise up 5 mph  |
+|   2   | Cruise down 5 mph |
+|   3   |   Cruise cancel   |
+|   q   |     Exit all      |