Add typing to jsonSerialize

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

lib/Activity/ChangeSet.php

@@ -69,15 +69,7 @@ class ChangeSet implements \JsonSerializable {
 		return $this->after;
 	}
 
-	/**
-	 * Specify data which should be serialized to JSON
-	 *
-	 * @link http://php.net/manual/en/jsonserializable.jsonserialize.php
-	 * @return mixed data which can be serialized by <b>json_encode</b>,
-	 * which is a value of any type other than a resource.
-	 * @since 5.4.0
-	 */
-	public function jsonSerialize() {
+	public function jsonSerialize(): array {
 		return [
 			'before' => $this->getBefore(),
 			'after' => $this->getAfter(),