Doc: minor language corrections

Task-number: QTBUG-35725
Change-Id: I84e47a72487b57665d0c11066cf73112a74a3529
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
doc: fixed broken links in qtdoc

Fixed: core.qdoc
       addons.qdoc

Task-number: QTBUG-34749
Change-Id: I77fea8d449a491c7d595f57704ce1886e61c6c38
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>