Fix argument spelling in uninstantiated vec_t function

[Diego Novillo]

Caught by Clang (which also checks uninstantiated templates).

        PR bootstrap/54484
        * vec.h (vec_t::lower_bound): Fix spelling of LESSTHAN
        argument.
    
diff --git a/gcc/vec.h b/gcc/vec.h
index 441c9b5..fbf95d2 100644
--- a/gcc/vec.h
+++ b/gcc/vec.h
@@ -1075,7 +1075,7 @@ vec_t<T>::lower_bound (T obj, bool (*lessthan)(T, T)) 
const
 template<typename T>
 unsigned
 vec_t<T>::lower_bound (const T *ptr,
-                      bool (*lessthan_)(const T *, const T *)) const
+                      bool (*lessthan)(const T *, const T *)) const
 {
   unsigned int len = VEC_length (T, this);
   unsigned int half, middle;