fix typo in Bragg's law

src/10_xray.md

@@ -441,7 +441,7 @@ $$
 \left|\mathbf{k'}-\mathbf{k} \right|^2=\left|\mathbf{G_{hkl}}\right|^2 \\
 2k^2-2\mathbf{k'} \cdot \mathbf{k} = G_{hkl}^2 \\
 2k^2-2 \left(\mathbf{k}+\mathbf{G_{hkl}}\right) \cdot \mathbf{k} = G_{hkl}^2 \\
-2 k \cos(\phi) = G_{hkl}^2
+2 k \cos(\phi) = G_{hkl}
 $$
 
 Where we use $|\mathbf{k'}| = |\mathbf{k}|$ in the second line and insert the Laue condition in the third line. In the exercises this week, you will see that Miller planes $(hkl)$ are normal to $G_{hkl}$ vectors and that the distance between planes is given by $d_{hkl} = \frac{2 \pi}{G_{hkl}}$. With this, one can finally derive **Bragg's Law**: