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 
`Allow editing` to me means that I'll be able to modify the file online. Is that the message you want to convey?