Update SECURITY.md (#11971)

Fix wording around how Microsoft's security policy applies to this .NET Foundation code repository.
Same idea as https://github.com/NuGet/NuGetGallery/pull/9164

SECURITY.md

@@ -4,7 +4,7 @@
 
 Microsoft takes the security of our software products and services seriously, which includes all source code repositories managed through our GitHub organizations, which include [Microsoft](https://github.com/Microsoft), [Azure](https://github.com/Azure), [DotNet](https://github.com/dotnet), [AspNet](https://github.com/aspnet), [Xamarin](https://github.com/xamarin), and [our GitHub organizations](https://opensource.microsoft.com/).
 
-If you believe you have found a security vulnerability in any Microsoft-owned repository that meets [Microsoft's definition of a security vulnerability](https://aka.ms/opensource/security/definition), please report it to us as described below.
+Microsoft serves as the primary maintainer of this repository. If you believe you have found a security vulnerability that meets [Microsoft's definition of a security vulnerability](https://aka.ms/opensource/security/definition), please report it to us as described below.
 
 ## Reporting Security Issues