[gnome]change default screenshot location

Please provide the output for:

gsettings get org.gnome.gnome-screenshot auto-save-directory

When providing terminal output, please wrap the text in triple backticks (```). Thet ia looks like:

```
the
text
```

This will cause it to be rendered like this:

the
text

making it much easier to read and work with for those trying to help.

Please also see How to provide good information