Update rescript-app command to use latest version

Added the missing `@latest` suffix to the pnpm command, to use the latest version.

I don’t know for yarn nor bun as I don’t use them.

Author: laurentpayot

README.md

@@ -15,7 +15,7 @@ yarn create rescript-app
 or
 
 ```sh
-pnpm create rescript-app
+pnpm create rescript-app@latest
 ```
 
 or