Recommend installing exact RC (#6944)

Author: eps1lon

src/content/blog/2024/04/25/react-19-upgrade-guide.md

@@ -70,7 +70,13 @@ We expect most apps will not be affected since the transform is enabled in most
 To install the latest version of React and React DOM:
 
 ```bash
-npm install react@rc react-dom@rc
+npm install --save-exact react@rc react-dom@rc
+```
+
+Or, if you're using Yarn:
+
+```bash
+yarn add --exact react@rc react-dom@rc
 ```
 
 If you're using TypeScript, you also need to update the types. Once React 19 is released as stable, you can install the types as usual from `@types/react` and `@types/react-dom`.  Until the stable release, the types are available in different packages which need to be enforced in your `package.json`: