Fix comment in ftp-nb-fget.xml: file is opened for writing (GH-3846)

Author: leonardolara

reference/ftp/functions/ftp-nb-fget.xml

@@ -110,7 +110,7 @@
 <![CDATA[
 <?php
 
-// open some file for reading
+// open some file for writing
 $file = 'index.php';
 $fp = fopen($file, 'w');