Fix typo "overfow" to "overflow" [ci skip]

include/mruby.h

@@ -287,7 +287,7 @@ typedef struct mrb_state {
   struct RObject *nomem_err;              /* pre-allocated NoMemoryError */
   struct RObject *stack_err;              /* pre-allocated SysStackError */
 #ifdef MRB_GC_FIXED_ARENA
-  struct RObject *arena_err;              /* pre-allocated arena overfow error */
+  struct RObject *arena_err;              /* pre-allocated arena overflow error */
 #endif
 
   void *ud; /* auxiliary data */