Lession 26: CardTheme needs CardThemeData

In Lesson 26 Shaun assigns the character card to the theme file like this:
```
cardTheme: CardTheme (
...
)
```
This is deprecated. The error you'll be seeing is something like this:
The argument type 'CardTheme' can't be assigned to the parameter type 'CardThemeData?'

As of the current Flutter version (Flutter 3.22+) this needs to be changed to the following to be working:
```
cardTheme: CardThemeData (
...
)
```

Change suggestion courtesy of ChatGPT.