Update FSharpHighlightUsagesExtension.fs

Partial fix for #351, matches logic in https://github.com/mono/monodevelop/blob/10ed4e4e8806fc41192ca8db1a50f4724c7c1b6e/main/src/addins/MonoDevelop.SourceEditor2/MonoDevelop.SourceEditor/AbstractUsagesExtension.cs#L165