fix the build issue of F# example

[syohex]

Fixes

F# example cannot be built with latest Termina.Gui. I got the following error when building.

% cd FSharpExample
% dotnet build
...
Build FAILED.

/home/syohei/tmp/Terminal.Gui/FSharpExample/Program.fs(22,18): error FS0039: The type 'Button' does not define the field, constructor or member 'Accept'. [/home/syohei/tmp/Terminal.Gui/FSharpExample/FSharpExample.fsproj]
    0 Warning(s)
    1 Error(s)

The property name was updated at 9990a55 however F# example has not been updated yet.

Proposed Changes/Todos

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working