Updated the PO Documentation part to reflect the last changes

Author: ahmed-rashad-alnaggar

README.md

@@ -158,10 +158,10 @@ $translations = [
 
 $dumper = new PoFileDumper;
 
-$dumper->dump($translations, 'path/to/translations/file.po', ['metadata' => [
+$dumper->dump($translations, 'path/to/translations/file.po', [
     'Language' => 'ar'
     'Plural-Forms' => 'nplurals=6; plural=n==0 ? 0 : n==1 ? 1 : n==2 ? 2 : n % 100 >= 3 && n % 100 <= 10 ? 3 : n % 100 >= 11 ? 4 : 5;'
-]]);
+]);
 ```
 
 ## XLIFF