[101] Wrong SizedBox height in description

[bapspatil]

On page https://codelabs.developers.google.com/codelabs/mdc-101-flutter/#3, under the heading Add the TextField widgets:

Add two new text fields and a spacer after SizedBox(height: 120.0).

should be:

Add two new text fields and a spacer after SizedBox(height: 12.0).