[guide] [react] Update React README ref to jsx-a11y/img-has-alt

https://github.com/evcohen/eslint-plugin-jsx-a11y/pull/220

Author: TrevorSayre

react/README.md

@@ -271,7 +271,7 @@
     />
     ```
 
-  - Always include an `alt` prop on `<img>` tags. If the image is presentational, `alt` can be an empty string or the `<img>` must have `role="presentation"`. eslint: [`jsx-a11y/img-has-alt`](https://github.com/evcohen/eslint-plugin-jsx-a11y/blob/master/docs/rules/img-has-alt.md)
+  - Always include an `alt` prop on `<img>` tags. If the image is presentational, `alt` can be an empty string or the `<img>` must have `role="presentation"`. eslint: [`jsx-a11y/alt-text`](https://github.com/evcohen/eslint-plugin-jsx-a11y/blob/master/docs/rules/alt-text.md)
 
     ```jsx
     // bad