Wording fixes for sharing dialog

* change 'update' to more understandable 'change', fix #10408 @farson2003
* change 'Allow Public Upload' to more understandable 'Allow editing', fix #10438 @octlabs

Please review @owncloud/designers @schiesbn 
It’s in the section, and nowhere do we say »public« anyway. So it’s clearer or at least as clear as before.