Update flutter.mdx (#1840)

Typo: fix formatting

Co-authored-by: myronkaifung <97630035+myronkaifung@users.noreply.github.com>

Author: alkhatibdev

pages/docs/tracking-methods/sdks/flutter.mdx

@@ -581,7 +581,7 @@ The simplest way is to use our [sample nginx config](https://github.com/mixpanel
 **Step 2: Point Flutter SDK at your server**
 Add the following line, replacing `YOUR_PROXY_DOMAIN` with your proxy server's domain.
 
-**Example Usage
+**Example Usage**
 ```dart
 // route data to your proxy server
 mixpanel.setServerURL("https://<YOUR_PROXY_DOMAIN>");
@@ -596,4 +596,4 @@ mixpanel.init("<YOUR_PROJECT_TOKEN>", {api_host: "https://<YOUR_PROXY_DOMAIN>"})
 
 ## Release History
 
-[See all releases.](https://github.com/mixpanel/mixpanel-flutter/releases)
+[See all releases.](https://github.com/mixpanel/mixpanel-flutter/releases)