[libc++][nfc] Remove double spaces.

[Mordante]

Summary

Based on the comment of @Quuxplusone in D111961. It seems no tests are
affected, but give it a run on the CI to be sure.

libcxx/include/vector

#endif
     void __annotate_shrink(size_type __old_size) const _NOEXCEPT
     {
       __annotate_contiguous_container(data(), data() + capacity(),
                                       data() + __old_size, data() + size());
     }
 
   struct _ConstructTransaction {
     explicit _ConstructTransaction(vector &__v, size_type __n)
-      : __v_(__v),  __pos_(__v.__end_), __new_end_(__v.__end_ + __n) {
+      : __v_(__v), __pos_(__v.__end_), __new_end_(__v.__end_ + __n) {
 #ifndef _LIBCPP_HAS_NO_ASAN
       __v_.__annotate_increase(__n);
 #endif
     }
     ~_ConstructTransaction() {
       __v_.__end_ = __pos_;
 #ifndef _LIBCPP_HAS_NO_ASAN
       if (__pos_ != __new_end_) {

 }
 
 template <class _Tp, class _Allocator>
 typename vector<_Tp, _Allocator>::iterator
 vector<_Tp, _Allocator>::erase(const_iterator __first, const_iterator __last)
 {
 #if _LIBCPP_DEBUG_LEVEL == 2
     _LIBCPP_ASSERT(__get_const_db()->__find_c_from_i(_VSTD::addressof(__first)) == this,
-        "vector::erase(iterator,  iterator) called with an iterator not"
+        "vector::erase(iterator, iterator) called with an iterator not"
         " referring to this vector");
     _LIBCPP_ASSERT(__get_const_db()->__find_c_from_i(_VSTD::addressof(__last)) == this,
-        "vector::erase(iterator,  iterator) called with an iterator not"
+        "vector::erase(iterator, iterator) called with an iterator not"
         " referring to this vector");
 #endif
     _LIBCPP_ASSERT(__first <= __last, "vector::erase(first, last) called with invalid range");
     pointer __p = this->__begin_ + (__first - begin());
     if (__first != __last) {
         this->__destruct_at_end(_VSTD::move(__p + (__last - __first), this->__end_, __p));
         this->__invalidate_iterators_past(__p - 1);
     }