Wrong anchor link for Documentation in ReadMe

[soheilrahsaz]

Hello
In the Readme file in the table of contents, there is a link for documentation:
Documentation
but it seems to be wrong and does not work and should be
https://github.com/ionicfirebaseapp/getwidget#flutter-getwidget-documentation
instead
(it's #documentation but should be #flutter-getwidget-documentation).

The same issue is in the pub.dev link too
https://pub.dev/packages/getwidget

[ashwinnavange]

fixed in #349