Fix unexpected delimiter at start of line in to_hex formatter

[starsunyzl]

When formatting as single line, there is an extra space at the beginning of the line. I think it's unexpected, not by design.

std::vector<int> vec{1, 2, 3, 4, 5, 6, 7};

// single line
spdlog::info("[{:n}]", spdlog::to_hex(vec.begin(), vec.end()));

// multi-line
spdlog::info("[{}]", spdlog::to_hex(vec.begin(), vec.end(), 3));
spdlog::info("[{:a}]", spdlog::to_hex(vec.begin(), vec.end(), 3));

[gabime]

Thanks @starsunyzl