docs: remove extra word in grammar (Redocly#1539)

README.md

@@ -323,7 +323,7 @@ Redoc.init(specOrSpecUrl, options, element, callback?)
 - `specOrSpecUrl` is either JSON object with specification or an URL to the spec in `JSON` or `YAML` format
 - `options` [options object](#redoc-options-object)
 - `element` DOM element to put ReDoc into
-- `callback` (optional) - callback to be called after Redoc has been fully rendered. It is also called also on errors with error as the first argument
+- `callback` (optional) - callback to be called after Redoc has been fully rendered. It is also called on errors with error as the first argument
 
 ```js
 Redoc.init('http://petstore.swagger.io/v2/swagger.json', {