Fix capitalization of Nextcloud

Summary: Nextcloud is not written like NextCloud

Test Plan: There is no more instance of 'NextCloud' in the codebase.

Reviewers: nicolasfella, apol

Reviewed By: nicolasfella, apol

Subscribers: romangg, pino, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D23224