typo fix

src/10_xray.md

@@ -471,7 +471,7 @@ Computing the structure factor allows us to resolve this apparent contradiction.
     In this case, each reciprocal lattice point gives one interference peak, none of which are absent.
 
 
-As a demonstration of how it happesn, let us compute the structure factor of the FCC lattice using the conventional unit cell in the real space.
+As a demonstration of how it happens, let us compute the structure factor of the FCC lattice using the conventional unit cell in the real space.
 
 ![](figures/fcc_mod.svg)