Incorrect return value for fmt::format shown in the development docs

[vitaut]

There should be std::string instead of std::wstring in

std::wstring fmt::format(StringRef format_str, ArgList args)

http://cppformat.github.io/dev/api.html#_CPPv2N3fmt6formatE9StringRef7ArgList

[vitaut]

This is a bug in Doxygen 1.7.6.1:

        <type>std::string</type>
        <definition>std::wstring fmt::format</definition>
        <argsstring>(StringRef format_str, ArgList args)</argsstring>

[vitaut]

Fixed in 5a61d54.