Fix query builder code examples

The code examples included multiple queries in
the `union` and `addUnion`, which is not possible

Author: BackEndTea

src/Query/QueryBuilder.php

@@ -515,7 +515,7 @@ public function forUpdate(ConflictResolutionMode $conflictResolutionMode = Confl
      *
      * <code>
      *     $qb = $conn->createQueryBuilder()
-     *         ->union('SELECT 1 AS field1', 'SELECT 2 AS field1');
+     *         ->union('SELECT 1 AS field1');
      * </code>
      *
      * @return $this
@@ -537,7 +537,8 @@ public function union(string|QueryBuilder $part): self
      * <code>
      *     $qb = $conn->createQueryBuilder()
      *         ->union('SELECT 1 AS field1')
-     *         ->addUnion('SELECT 2 AS field1', 'SELECT 3 AS field1')
+     *         ->addUnion('SELECT 2 AS field1')
+     *         ->addUnion('SELECT 3 AS field1');
      * </code>
      *
      * @return $this