Fix theta and phi

docs/challenge/challenge.md

@@ -70,7 +70,7 @@ rotation for later reference. This part has two sub-parts: A and B.
 ##### Part 3A
 
 Please write down the math you propose to implement to rotate the PMTs
-over two angles: &theta and &phi, where &theta is the polar
+over two angles: θ and φ, where θ is the polar
 angle of a system of spherical coordinates with its zenith direction
 along the z-axis, and &phi is the azimuthal angle in the same
 system.
@@ -79,7 +79,7 @@ system.
 
 It is suggested to implement two functions that do the work of the
 rotation, first to obtain the parameters of the rotation from the
-angles &theta and &phi, and another to apply the rotation to the
+angles θ and φ, and another to apply the rotation to the
 direction of the PMTs selected in part 2.
 
 ### Testing