Update Table.php

You must not open a <th> and close it as a </td>.
read http://www.w3schools.com/tags/tag_th.asp

Author: Franz Holzinger

src/PhpWord/Writer/HTML/Element/Table.php

@@ -51,7 +51,7 @@ public function write()
                     $cellTag = $tblHeader ? 'th' : 'td';
                     $content .= "<{$cellTag}>" . PHP_EOL;
                     $content .= $writer->write();
-                    $content .= '</td>' . PHP_EOL;
+                    $content .= "</{$cellTag}>" . PHP_EOL;
                 }
                 $content .= '</tr>' . PHP_EOL;
             }