Fix tab heading levels in toast.md to use # instead of ### for screenshot tabs

The screenshot tabs used ### (H3) heading level, same as the platform-specific
tabs in the included toast-snackbar-setup.md file. This caused the Microsoft
Learn tab system to confuse the two tab groups, resulting in multiple tabs
appearing selected simultaneously. Changed to # (H1) to match the working
pattern used in snackbar.md.

Co-authored-by: jfversluis <939291+jfversluis@users.noreply.github.com>

Author: Copilot

docs/maui/alerts/toast.md

@@ -40,11 +40,11 @@ await toast.Show(cancellationTokenSource.Token);
 When calling `Toast.Make()`, its parameter `string text` is required. All other parameters are optional. Its optional parameter `ToastDuration duration` uses the default duration of `ToastDuration.Short`. Its optional parameter `double fontSize` uses the default value of `14.0`.
 
 The following screenshot shows the resulting Toast:
-### [Android](#tab/android)
+# [Android](#tab/android)
 
 ![Screenshot of an Toast on Android](../images/alerts/toast-android.gif "Toast on Android")
 
-### [iOS](#tab/ios)
+# [iOS](#tab/ios)
 
 ![Screenshot of an Toast on iOS](../images/alerts/toast-ios.gif "Toast on iOS")