Change doc for SectionedCardList

[djhuistra]

The documentation for using SectionedCardList was confusing for me.

It uses "GplayCardSection" and "MyCardSection" in the examples before it introduces (how to) extend the default "CardSection".

At first I believed "GplayCardSection" was part of the library and was confused as to why the example did not work.

By using the default "CardSection" in the examples, the code is directly runnable, which I believe is preferred :)