Update RC blog post to use rc3 (#3312)

Author: iamnapo

content/blog/2020-08-10-react-v17-rc.md

@@ -272,20 +272,20 @@ We encourage you to try React 17.0 Release Candidate soon and [raise any issues]
 To install React 17 RC with npm, run:
 
 ```bash
-npm install [email protected].2 [email protected].2
+npm install [email protected].3 [email protected].3
 ```
 
 To install React 17 RC with Yarn, run:
 
 ```bash
-yarn add [email protected].2 [email protected].2
+yarn add [email protected].3 [email protected].3
 ```
 
 We also provide UMD builds of React via a CDN:
 
 ```html
-<script crossorigin src="https://unpkg.com/[email protected].2/umd/react.production.min.js"></script>
-<script crossorigin src="https://unpkg.com/[email protected].2/umd/react-dom.production.min.js"></script>
+<script crossorigin src="https://unpkg.com/[email protected].3/umd/react.production.min.js"></script>
+<script crossorigin src="https://unpkg.com/[email protected].3/umd/react-dom.production.min.js"></script>
 ```
 
 Refer to the documentation for [detailed installation instructions](/docs/installation.html).