Java Interfaces: Fixed wording for clarification

Under the first code block it referred to only one field and two methods, when there two fields, two methods, and one default method. I also added a few missing words and rephrased one sentence. I hope this helps.

[x ] Added descriptive name to PR
- Your PR should NOT be called Update index.md, since it does not help the maintainer understand what has been changed.
- For example, if you create a Variables article inside the Python directory, the pull request title should be Python: add Variables article.
- Additional PR title examples:
- Git: edit Git Commit article
- PHP: create PHP section and add Data Structures article
[ x] Reviewed necessary formatting guidelines in CONTRIBUTING.md.
[ x] No plagiarized, duplicate, or repetitive content that has been directly copied from another source.