Remove unreferenced switching-physics-engine tutorial and fix broken …

…link in 04_physics_engines (#684)

Signed-off-by: Shameek Ganguly <[email protected]>
(cherry picked from commit c5f2954)

# Conflicts:
#	tutorials/04-switching-physics-engines.md

tutorials/04_physics_engines.md

@@ -1,3 +1,3 @@
 \page physicsengine Use different physics engines
 
-See [Physics engines tutorial](https://gazebosim.org/api/gazebo/3.8/physics.html) in Gazebo.
+See [Physics engines tutorial](https://gazebosim.org/api/sim/9/physics.html) in Gazebo.