Fix broken id anchor (#2095)

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**Summary**

Just a small fix :)

**Test Plan**

1. Check out draft-js repo
1. Install dependencies with `yarn`
1. Change working directory to the website dir: `cd website`
1. Launch website in development mode: `yarn start`
1. Click the **Docs** link in the header
1. Click the **Learn more about why** link at the bottom of the page
1. Verify the page **Issues and Pitfalls** opens and scrolls to the **Missing Draft.css** heading
Pull Request resolved: #2095

Differential Revision: D15821826

Pulled By: claudiopro

fbshipit-source-id: 85977433416c7642f477d5709bee70f0795cf313

Author: sajadtorkamani

docs/Overview.md

@@ -98,6 +98,6 @@ Because Draft.js supports unicode, you must have the following meta tag in the `
 <meta charset="utf-8" />
 ```
 
-`Draft.css` should be included when rendering the editor. Learn more about [why](/docs/advanced-topics-issues-and-pitfalls.html#missing-draft-css).
+`Draft.css` should be included when rendering the editor. Learn more about [why](/docs/advanced-topics-issues-and-pitfalls.html#missing-draftcss).
 
 Next, let's go into the basics of the API and learn what else you can do with Draft.js.