Modal documentation update

[prajkt-yeole]

• Operating System - WIndows 7 x64
• Browser - Firefox Quantum 57.0.3 64 bit

in the example of Vertically centered Modal documentation.

the class name modal-dialog-centered is not given along with modal-dialog

please check the link and attached image.

https://getbootstrap.com/docs/4.0/components/modal/#vertically-centered

[tuvokian]

Already fixed by #25111 & this issue has been raised before (#25178) - search before raising an issue :)

[Johann-S]

Closed thanks to @zacchaeusluke 👍