docs[okta-vue]: Updates README.md

[nountie]

Update example from the docs to comply with the vue/no-deprecated-router-link-tag-prop ESLint rule

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• [] Refactoring (no functional changes, no api changes)
• Adding Tests
• Build related changes
• CI related changes
• Documentation changes
• Other... Please describe:

What is the current behavior?

The example from the documentation is outdated, and if you use newer vue-router (3.1.0+, 4), the button will not render correctly. Here are more details.

Issue Number: N/A

What is the new behavior?

V-slot was used to recreate previous behavior (also with router-link-active and router-link-exact-active classes).

Does this PR introduce a breaking change?

• Yes
• No