Update README example

@@ -3,7 +3,7 @@
 Example:
 Example:
 ```dart
 ```dart
 final pdf = new PDFDocument();
 final pdf = new PDFDocument();
-final page = new PDFPage(pdf, pageFormat: new PDFPageFormat(PDFPageFormat.LETTER));
+final page = new PDFPage(pdf, pageFormat: PDFPageFormat.LETTER);
 final g = page.getGraphics();
 final g = page.getGraphics();
 final font = new PDFFont(pdf);
 final font = new PDFFont(pdf);