Fix package name in FastAPI README template

[DavidRV00]

fix #19480

Just using templated package name in the README template instead of hardcoded default package name.

To verify, simply run the FastAPI server generator with the command given in the issue, and see that the package name is filled in using the template.

PR checklist

• Read the contribution guidelines.
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  ./mvnw clean package || exit
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  (For Windows users, please run the script in Git BASH)
  Commit all changed files.
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