fix(10n): Fixed grammar

Reported at Transifex

Signed-off-by: rakekniven <2069590+rakekniven@users.noreply.github.com>
Signed-off-by: nextcloud-command <nextcloud-command@users.noreply.github.com>

Author: rakekniven

apps/theming/src/components/BackgroundSettings.vue

@@ -271,7 +271,7 @@ export default {
 		async applyFile(path) {
 			if (!path || typeof path !== 'string' || path.trim().length === 0 || path === '/') {
 				console.error('No valid background have been selected', { path })
-				showError(t('theming', 'No background have been selected'))
+				showError(t('theming', 'No background has been selected'))
 				return
 			}