docs: add missing .dropup in dropup centered example

[julien-deramond]

Description

Add missing .dropup for the dropup centered example.

Motivation & Context

The arrow is in the wrong direction; should point to the top.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

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
• I have added tests to cover my changes
• [w] All new and existing tests passed

Related issues

N/A

Live preview

[julien-deramond]

⚠️ Apparently there is a regression between c0f3036 and bb7664d. Testing it on the main branch gives:

• Centered: not centered
• Dropup centered: not centered and points to the bottom

Edit: see #35968

[mdo]

Fixed by #35973—no need for the second class now :). Thanks and good looking out! <3

[julien-deramond]

@mdo Please reconsider reopening it because IMO #35973 doesn't fix the issue with the direction of the dropup centered arrow.

With #35973 (https://deploy-preview-35973--twbs-bootstrap.netlify.app/docs/5.1/components/dropdowns/#dropup-centered) and so in the main branch now, the arrow is pointing down:

With this PR (and the .dropup class) (https://deploy-preview-35948--twbs-bootstrap.netlify.app/docs/5.1/components/dropdowns/#dropup-centered), the arrow direction is the same than for the other dropup examples (arrow is pointing up):

[GeoSot]

CC @mdo we may reconsider this