Update 10_xray.md - removed minus signs with unclear origin in structure factor

src/10_xray.md

@@ -197,7 +197,7 @@ $$
 \mathbf{G}&=h\mathbf{b}_1+k\mathbf{b}_2+l\mathbf{b}_3\\
 S&=f\left(\mathrm{e}^{i\mathbf{G}\cdot\mathbf{r}_1}+\mathrm{e}^{i\mathbf{G}\cdot\mathbf{r}_2}+\mathrm{e}^{i\mathbf{G}\cdot\mathbf{r}_3}+\mathrm{e}^{i\mathbf{G}\cdot\mathbf{r}_4}\right)\\
 &=f\left(1+\mathrm{e}^{i/2(h\mathbf{b}_1\cdot\mathbf{a}_1+k\mathbf{b}_2\cdot\mathbf{a}_2)}+\mathrm{e}^{i/2(k\mathbf{b}_2\cdot\mathbf{a}_2+l\mathbf{b}_3\cdot\mathbf{a}_3)}+\mathrm{e}^{i/2(h\mathbf{b}_1\cdot\mathbf{a}_1+l\mathbf{b}_3\cdot\mathbf{a}_3)}\right)\\
-&=f\left(1+\mathrm{e}^{-i\pi(h+k)}+\mathrm{e}^{-i\pi(k+l)}+\mathrm{e}^{-i\pi(h+l)}\right)
+&=f\left(1+\mathrm{e}^{i\pi(h+k)}+\mathrm{e}^{i\pi(k+l)}+\mathrm{e}^{i\pi(h+l)}\right)
 \end{aligned}
 $$