correction for Documentation about picamera

[waveform80]

Hi NicoLoco - thanks for the correction, and no probs on the method of communication - if the docs are wrong, then an issue is a perfect way to discuss the correction (incidentally you should be able to find my e-mail by clicking on "waveform80" at the top of the GitHub page).

On the subject of the ISO setting, I'll have to have a play with it again and see if I can get it to make a difference - last time I tried it definitely didn't do anything but there've been a few firmware updates since then. I'll try and find some time tonight to play with it.

Incidentally, in the next release, the default ISO value will become 0 (which is effectively "auto") instead of the original 400, to match the recent change in raspistill (setting exposure_mode doesn't do anything unless ISO is 0, which should cut down on some confusion for new users).