fix typo

Author: oscarlevin

source/sec_addtops-numbth.ptx

@@ -1024,7 +1024,7 @@
       <idx><h>linear Diophantine equation</h><see>Diophantine equation</see></idx>
       <p>
         An equation in two or more variables is called a <term>Diophantine equation</term> if only integer solutions are of interest.
-        A <term>linear</term> Diophantine equation takes the form <m>a_1x_1 + a_2x_x + \cdots + a_nx_n = b</m> for constants <m>a_1,\ldots, a_n, b</m>.
+        A <term>linear</term> Diophantine equation takes the form <m>a_1x_1 + a_2x_2 + \cdots + a_nx_n = b</m> for constants <m>a_1,\ldots, a_n, b</m>.
       </p>
 
       <p>