Fix typo in SystemTextFontNameProperty

[rstm-sf]

Hello!

Fix typo in SystemTextFontNameProperty

InvalidCastException: Unable to cast object of type 'System.String' to type 'System.Double'.
This exception was originally thrown at this call stack:
    WpfMath.Controls.FormulaControl.CoerceScaleValue(System.Windows.DependencyObject, object) in FormulaControl.xaml.cs
    System.Windows.DependencyObject.ProcessCoerceValue(System.Windows.DependencyProperty, System.Windows.PropertyMetadata, ref System.Windows.EntryIndex, ref int, ref System.Windows.EffectiveValueEntry, ref System.Windows.EffectiveValueEntry, ref object, object, object, System.Windows.CoerceValueCallback, bool, bool, bool)
    System.Windows.DependencyObject.UpdateEffectiveValue(System.Windows.EntryIndex, System.Windows.DependencyProperty, System.Windows.PropertyMetadata, System.Windows.EffectiveValueEntry, ref System.Windows.EffectiveValueEntry, bool, bool, System.Windows.OperationType)
    System.Windows.DependencyObject.SetValueCommon(System.Windows.DependencyProperty, object, System.Windows.PropertyMetadata, bool, bool, System.Windows.OperationType, bool)
    System.Windows.DependencyObject.SetValue(System.Windows.DependencyProperty, object)
    System.Windows.Baml2006.WpfMemberInvoker.SetValue(object, object)
    MS.Internal.Xaml.Runtime.ClrObjectRuntime.SetValue(System.Xaml.XamlMember, object, object)
    MS.Internal.Xaml.Runtime.ClrObjectRuntime.SetValue(object, System.Xaml.XamlMember, object)

https://github.com/ForNeVeR/wpf-math/blob/af5c330fecdaec39e85324292372228350724a2a/src/WpfMath/Controls/FormulaControl.xaml.cs#L180-L184

[ForNeVeR]

Thanks a lot!