Correct builder name

Author: glombek

17/umbraco-cms/extending/flag-providers.md

@@ -78,7 +78,7 @@ internal class MyDocumentFlagProvider : IFlagProvider
 The flag provider needs to be registered with Umbraco in a composer or application startup with:
 
 ```csharp
-  builder.FlagProviders()
+  builder.SignProviders()
     .Append<MyDocumentFlagProvider>();
 ```