Fix three parameter tags

[karpovantonme]

Three Doxygen tags that do not match the declaration below them. Documentation only.

local_is_dst, dst_rules.hpp

 *  @param dst_length_minutes length of dst adjusment
 */
static time_is_dst_result
local_is_dst(const date_type& current_day,
             ...
             const time_duration_type& dst_length)

The parameter is dst_length. The _minutes suffix looks like a leftover from when it was a count rather than a time_duration_type; the six tags above it all match.

gather_month_strings and gather_weekday_strings, strings_from_facet.hpp

 *@param charT The type of char to use when gathering typically char
 *             or wchar_t.
 *@param locale The locale to use when gathering the strings
 */
template<typename charT>
std::vector<std::basic_string<charT> >
gather_month_strings(const std::locale& locale, bool short_strings=true)

charT is the template parameter, so it wants @tparam. As written, Doxygen looks for a function argument called charT, does not find one, and the template parameter ends up undocumented.

Both are what clang -Wdocumentation reports as parameter '...' not found in the function declaration.