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