Update bottom border on dark navbar example

[julien-deramond]

Description

As mentioned in #38706, .border-bottom-dark doesn't exist in Bootstrap.

Moreover, adding a border is finally probably a bit useless here since it adds some noise to the understanding of the concept explained in this section. Rather than fixing it, IMO, it's better to completely remove the border and focus only on the color scheme concept.

However, if you have a strong opinion on its presence and that it's still useful to see the limit of the navbar in dark mode, we could use in this case .border-body instead of .border-dark.

Motivation & Context

A better understanding of the feature without noise.

Type of changes

• Refactoring (non-breaking change)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• (N/A) I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-38707--twbs-bootstrap.netlify.app/docs/5.3/components/navbar/#color-schemes

Related issues

Closes #38706

[julien-deramond]

Based on #38706 (comment), reintroduced the bottom border associated to .border-body for its color (0b2df3b).