Tools: add alt text to markdown images

Author: peterbarker

Tools/simulink/arducopter/README.md

@@ -2,7 +2,7 @@
 
 The Simulink file [arducopter.slx](https://github.com/fbredeme/ardupilot_simulink/blob/master/arducopter.slx) contains a model of the ArduCopter Stabilize and Althold controllers. The plant models of the copter itself can be generated by [system identification](https://ardupilot.org/copter/docs/systemid-mode.html).
 
-![](simulink_model.PNG?raw=true "Simulink screenshot")
+![Simulink screenshot](simulink_model.PNG)
 
 To run the simulation, the relevant data from a *.bin log file needs to be loaded to the MATLAB workspace. This can be done with the two scripts that are provided: