Fix typo in README

[bortsov]

Chapter "Using captures"

It should be

auto result = ctre::match<"(?<year>\\d{4})/(?<month>\\d{1,2})/(?<day>\\d{1,2})">(s);
return date{result.get<"year">(), result.get<"month">, result.get<"day">};

// or in C++ emulation, but the object must have a linkage
static constexpr ctll::fixed_string year = "year";
static constexpr ctll::fixed_string month = "month";
static constexpr ctll::fixed_string day = "day";
return date{result.get<year>(), result.get<month>(), result.get<day>()};

// or use numbered access
// capture 0 is the whole match
return date{result.get<1>(), result.get<2>(), result.get<3>()};

Otherwise code can't be compiled.