Update README.md

trick_sims/SIM_aircraft/README.md

@@ -170,7 +170,7 @@ Notice (in the figure) that $\psi_{diff}$ is in the direction that we want to tu
 
 Do you ever have to turn more than 𝜋 radians (180 degrees) to face any direction? No, you don’t.
 
-Let’s now define $\pis_{error}$ as the actual direction of turning.
+Let’s now define $\psi_{error}$ as the actual direction of turning.
 
 ![](images/Equation_9.png)
 
@@ -193,4 +193,4 @@ Like our desired speed, just we want something doesn’t mean that the aircraft
 But we need that total force in world coordinates so we can move our aircraft around in the world.
 So, if our heading of our aircraft is 𝜓, what is our direction vector expressed in world coordinates? 
 
-![](images/Equation_12.png)
+![](images/Equation_12.png)