Simplify benchmark inage link

ref_app/src/app/benchmark/readme.md

@@ -53,7 +53,7 @@ the $8$-bit MICROCHIP(R) AVR controller of the Arduino
 and the $32$-bit ARM(R) 8 controller
 of the BeagleBone Black Edition, Rev. C.
 
-![](https://github.com/ckormanyos/real-time-cpp/blob/master/ref_app/src/app/benchmark/images/app_benchmark_pi_agm.pdf)
+![](images/app_benchmark_pi_agm.pdf)
 
 The $\pi$ calculation requires approximately
 $470~\text{ms}$ and $1.5~\text{ms}$,