Fix the template parameter name in buffers_iterator_type

[karpovantonme]

One @tparam naming something the template does not take.

/** Type alias for the iterator type of a buffer sequence type.
    ...
    @tparam T The buffer sequence type to use.
*/
template <class BufferSequence>
using buffers_iterator_type = __see_below__;

The three other @tparam tags in this file, on buffers_type and its neighbours, all say BufferSequence, so this is the odd one out rather than a convention I am changing.

Not touched, in case you want it: http/message.hpp:284 documents @param args on a constructor taking Arg1&& arg1, ArgN&&... argn. Describing a pack collectively reads fine to me, and clang -Wdocumentation disagrees, so I left it as your call.