fix namespace

Author: troosan

src/PhpWord/Writer/Word2007/Element/Field.php

@@ -64,7 +64,7 @@ public function write()
         $xmlWriter->endElement(); // w:r
 
         if ($element->getText() != null) {
-            if ($element->getText() instanceof PhpOffice\PhpWord\Element\TextRun) {
+            if ($element->getText() instanceof \PhpOffice\PhpWord\Element\TextRun) {
 
                 $containerWriter = new Container($xmlWriter, $element->getText(), true);
                 $containerWriter->write();