Fix typo

include/math/matrix/forward.h

@@ -12,8 +12,8 @@ namespace math { namespace vector {
 
 namespace math { namespace matrix { namespace concat_strategy {
   struct {
-  } horizonal;
-  using horizontal_concat_t = decltype(horizonal);
+  } horizontal;
+  using horizontal_concat_t = decltype(horizontal);
   struct {
   } vertical;
   using vertical_concat_t = decltype(vertical);