Fix - to _ in Packaged applications doc (#7571)

## Summary

Small stale/typo char in docs when generating a project

```
➜ ntp -v uv-test && uv venv --python 3.12 --seed && uv init --app --package example-packaged-app
Directory /tmp/testing/uv-test created and switched to.
Using Python 3.12.4 interpreter at: /Users/coffee/.local/share/mise/installs/python/3.12/bin/python3.12
Creating virtual environment with seed packages at: .venv
 + pip==24.2
Activate with: source .venv/bin/activate.fish
Virtual environment created with Python 3.12 and activated.
Using Python 3.12.4 interpreter at: /Users/coffee/.local/share/mise/installs/python/3.12/bin/python3.12
Creating virtual environment with seed packages at: .venv
 + pip==24.2
Activate with: source .venv/bin/activate.fish
Initialized project `example-packaged-app` at `/private/tmp/testing/uv-test/example-packaged-app`

/tmp/testing/uv-test via  pyenv (uv-test) on ☁  (us-east-2)
➜ tree
   0 B    ┌─ README.md
4096 B    ├─ pyproject.toml
4096 B    │     ┌─ __init__.py
4096 B    │  ┌─ example_packaged_app
4096 B    ├─ src
8192 B ┌─ example-packaged-app
8192 B uv-test
```

## Test Plan

Eyeballs

docs/concepts/projects.md

@@ -224,7 +224,7 @@ example-packaged-app
 ├── README.md
 ├── pyproject.toml
 └── src
-    └── example_packaged-app
+    └── example_packaged_app
         └── __init__.py
 ```