docs: Minor updates to steps 3 & 4 in Klondike tutorial

[gilescm]

Description

I have just finished steps 1-4 of the Klondike tutorial and noticed a couple of minor things that look to be outdated with the current dart / flame versions.

Step 3 - late keyword isn't required

When following this step the analyser hightlight this rule unnecessary_late (introduced in Dart 2.16.0) which states:

Top-level and static variables with initializers are already evaluated lazily as if they are marked late.

I updated the notes around why the laziness is important so the reader can still understand the reasoning.

Step 4 - local _isDragging vs DragCallbacks.isDragged

Looking through the DragCallbacks mixin I found that onDragStart() and onDragEnd() both set a private _isDragged flag that acts the same as the _isDragging local flag created in the tutorial. This was added in #2472 along with @mustCallSuper annotations on both methods.

So this change brings the docs up to date by having the new onDragStart/onDragEnd methods reference their super's super.onDragStart and super.onDragEnd instead of the local flag, and now onDragUpdate, onDragEnd use if (!isDragged) { .. } instead of if (!_isDragging) { .. }.

Thanks for awesome tutorial! It's really well written and explains everything just as you need it without being overwhelming.

Checklist

• I have followed the Contributor Guide when preparing my PR.
• [-] I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• [-] I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

[spydon]

Just some trailing spaces that needs to be removed, otherwise it looks good to me.
Thanks for your contribution!

doc/tutorials/klondike/step3.md:58:76 MD009/no-trailing-spaces Trailing spaces [Expected: 0 or 2; Actual: 1]
doc/tutorials/klondike/step3.md:59:76 MD009/no-trailing-spaces Trailing spaces [Expected: 0 or 2; Actual: 1]
doc/tutorials/klondike/step3.md:60:75 MD009/no-trailing-spaces Trailing spaces [Expected: 0 or 2; Actual: 1]
doc/tutorials/klondike/step3.md:61:76 MD009/no-trailing-spaces Trailing spaces [Expected: 0 or 2; Actual: 1]
doc/tutorials/klondike/step4.md:585:97 MD009/no-trailing-spaces Trailing spaces [Expected: 0 or 2; Actual: 1]
doc/tutorials/klondike/step4.md:586:94 MD009/no-trailing-spaces Trailing spaces [Expected: 0 or 2; Actual: 1]

[gilescm]

@spydon Ah you beat me to it! I hadn't set up markdown linting on my fork yet sorry about that I'm pretty new to open source contributing, should be fixed now.

Thanks for the fast review 🙏