fix warning

Author: troosan

tests/PhpWord/Writer/Word2007/Part/DocumentTest.php

@@ -56,7 +56,7 @@ public function testWriteCustomProps()
         $docInfo->setCustomProperty('key6', new \DateTime());
         $docInfo->setCustomProperty('key7', time(), DocInfo::PROPERTY_TYPE_DATE);
 
-        $doc = TestHelperDOCX::getDocument($phpWord);
+        TestHelperDOCX::getDocument($phpWord);
 
 //         $this->assertTrue($doc->elementExists('/Properties/property[name="key1"]/vt:lpwstr'));
 //         $this->assertTrue($doc->elementExists('/Properties/property[name="key2"]/vt:bool'));