Super simple function to include. Really not sure why its missing from the Corona SDK....

function string.capital( str )
local firstLetter = string.sub(string.upper(str), 1, 1);
local otherLetters = string.sub(str, 2);

return firstLetter..otherLetters;
end

Maybe because Ansca isn't making the standart LUA Functions.
But this can be also done this way

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function firstToUpper(str)
    return (str:gsub("^%l", string.upper))
end