fixes typo

src/10_xray_solutions.md

@@ -162,6 +162,7 @@ S(\mathbf{G}) =
       f_1+f_2 & \text{if $h+k+l$ is even}\\
       f_1-f_2 & \text{if $h+k+l$ is odd}
 \end{cases}   
+$$
 
 4.
 Due to bcc systematic absences, the peaks from lowest to largest angle are: