The image that you are returning is in UINT8 (integer values from 0 to 255). To plot the bands as a true-colour image, the function reads values from 0 to 1. So, to convert the range from 0-255 to 0-1 you divide your values by 255.
However, by plotting the entire range of values, the images often appear quite dark (vegetation, soil, water etc. have a low reflectance). To improve the image and make it “brighter” you can multiply it by a factor, here (3.5).
Of course, you can adjust this to suit your needs.