Fix typo

README.md

@@ -66,7 +66,7 @@ Awesome Wagtail [![Awesome](https://cdn.rawgit.com/sindresorhus/awesome/d7305f38
 - [Wagtail Froala](https://github.com/jaydensmith/wagtailfroala) - Extends Wagtail to use the Froala WYSIWYG editor in RichTextField/RichTextBlock.
 - [Wagtail Medium Editor](https://github.com/dperetti/Django-wagtailmedium) - A customizable Medium Editor for Wagtail, with link anchors support.
 - [WagtailDraftail](https://github.com/springload/wagtaildraftail) – Draft.js editor for Wagtail, built upon [Draftail](https://github.com/springload/draftail) and [draftjs_exporter](https://github.com/springload/draftjs_exporter).
-- [wagtail-readinglevel](https://github.com/vixdigital/wagtail-readinglevel) – Wagtail plugin to determine the reading level of of text input into a rich text field.
+- [wagtail-readinglevel](https://github.com/vixdigital/wagtail-readinglevel) – Wagtail plugin to determine the reading level of text input into a rich text field.
 
 ### Widgets