Example does not user Sliver for Header/Footer

[Sheltek]

Change log for version 2.0.0 details breaking issue where header and footer only accept Sliver Widget. Example code uses Text() Widget. As a result the example code does not run and causes dart framework to crash. Please correct example code.

I only found this b/c another user in an unrelated Stack Overflow post mentioned not using Sliver in a custom scroll view caused a similar error. Then I stumbled across the change log and made the connection. Other devs may not be so lucky.