The line where you calculate tempo is wrong. It should be
tempo(examples) = ((theta' * X(examples, :)') - y(examples)) * X(examples,t)
Also try using matrix operations in MATLAB. Your code will be faster and it will also be easier to understand. For example, you can replace your nested loop with
E = X * theta - y;
for t = 1:thetas
result(t) = sum(E.*X(:,t));
end
You can replace your subsequent two loop for updating theta into one line
theta = theta - alpha * (1/m) * result';