update wording of email field

fixes #7505

cc @jancborchardt @PVince81 

Maybe one of you want to review (I mention you because you recently added changes to owncloud):
@ganomi @elchi @McNetic @miicha @ccerrillo @jbtbnl @MartialGeek @vsapronov @jcfischer @zombiehugs @AxelRb @GuillaumeAmat @petemcfarlane @frisco82 @ogasser @leo-b @tripflex @pellaeon @nickvergessen 

We need two thumbs up to merge into core. Thumbs up can be done by "colon"+1"colon". 
How to review: Check out this pull request and test if it works.
```
git fetch origin
git checkout -b email-field-wording origin/email-field-wording
```