Update 25_api_proposal.md (#35321)

This always drove me crazy

.github/ISSUE_TEMPLATE/25_api_proposal.md

@@ -25,6 +25,7 @@ namespace Microsoft.AspNetCore.Http
     {
 +       public Task WriteAsync(this HttpResponse response, StringBuilder builder);
     }
+}
 ```
 You may find the [Framework Design Guidelines](https://github.com/dotnet/runtime/blob/master/docs/coding-guidelines/framework-design-guidelines-digest.md) helpful.
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