Merge pull request #238 from TheLittleBoy/patch-2

代码编写错误修正

source/chapter2/12_Subscripts.md

@@ -112,7 +112,7 @@ struct Matrix {
         }
         set {
             assert(indexIsValidForRow(row, column: column), "Index out of range")
-            grid[(row * columns) + columns] = newValue
+            grid[(row * columns) + column] = newValue
         }
     }
 }