React Router: Correct terminology and fix punctuation

[Maddily]

Because

The lesson incorrectly uses "tag" to describe the index property, and a missing period affects readability. This PR fixes these issues.

This PR

-Corrects the use of "tag" to "property" when describing the index configuration in React Router.
-Adds a missing period to improve sentence structure.

Issue

Closes #XXXXX

Additional Information

Pull Request Requirements

• I have thoroughly read and understand The Odin Project curriculum contributing guide
• The title of this PR follows the location of change: brief description of change format, e.g. Intro to HTML and CSS lesson: Fix link text
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• If this PR addresses an open issue, it is linked in the Issue section
• If any lesson files are included in this PR, they have been previewed with the Markdown preview tool to ensure it is formatted correctly
• If any lesson files are included in this PR, they follow the Layout Style Guide