[doc] Fix the template parameter names in three Doxygen blocks

[karpovantonme]

Three Doxygen blocks name a template parameter the declaration does not have. Comments only.

accumulators/count.hpp

  @tparam T C++ builtin arithmetic type (integer or floating point).
  @tparam ThreadSafe Set to true to make increments and adds thread-safe.
*/
template <class ValueType, bool ThreadSafe>
class count {

ThreadSafe is right, T should be ValueType.

weight.hpp and sample.hpp

  @tparam Underlying arithmetic type.
*/
template <class T>
struct weight_type {

"Underlying" reads as an adjective here, but Doxygen takes the first word after @tparam as the name, so it documents a parameter called Underlying and reports T as undocumented. Inserting the name leaves the sentence as it was: @tparam T Underlying arithmetic type. Same in sample.hpp.

I checked the rest of accumulators/ and axis/ for the same thing; count.hpp is the only one there, the axis blocks all match their declarations.

Not included

A sweep of the whole library also flags axis/traits.hpp, histogram.hpp, indexed.hpp, unsafe_access.hpp and the five utility/*_interval.hpp headers, but every one of those is my parser failing on auto&& parameters and on operator(), not a real mismatch — I read them and the docs are correct. Mentioning it so nobody repeats the check and thinks there is more here.