Clarify package name on Integration Testing page

[jarrodcolburn]

In step 4 the package is referred to as introduction *:

website/src/content/cookbook/testing/integration/introduction.md

Line 160 in 81671a5

import 'package:introduction/main.dart';

In step 3 the the package is referred to as 'counter_app'

website/src/content/cookbook/testing/integration/introduction.md

Line 139 in 81671a5

counter_app/

In step 1 the package is not given a name in the flutter create command

website/src/content/cookbook/testing/integration/introduction.md

Line 33 in 81671a5

test the counter app produced by the `flutter create`

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Recommend:

updating step 1's flutter create to something like flutter create introduction_to_integration to give explicit package name. Then updating step 3's file tree, and step 4's test file to match.

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*there is an introduction package on pub.dev, so this could be confusing

[jarrodcolburn]

Sorry this is a pedantic issue. I would normally try to create the PR for it, but I see you're using a preprocessor and so I kinda want to see how you guys do it. Also, my other PR may not even be right now that I think about it.