chore(psalm): Add type hints to make psalm happy

Signed-off-by: Julius Härtl <jus@bitgrid.net>

lib/Activity/DescriptionSetting.php

@@ -29,7 +29,7 @@ class DescriptionSetting extends Setting {
 	 * @return string Lowercase a-z and underscore only identifier
 	 * @since 11.0.0
 	 */
-	public function getIdentifier() {
+	public function getIdentifier(): string {
 		return 'deck_card_description';
 	}
 
@@ -37,7 +37,7 @@ class DescriptionSetting extends Setting {
 	 * @return string A translated string
 	 * @since 11.0.0
 	 */
-	public function getName() {
+	public function getName(): string {
 		return $this->l->t('A <strong>card description</strong> inside the Deck app has been changed');
 	}
 }