docs(*): corrects async/await example that uses transactions

[deezone]

Fixes #11758

Pull Request check-list

• Documentation update

Description of change

This PR fixes a code example in the documentation under:

Advance Topics -> Migration Skeleton -> "The next is an example of a migration that has uses async/await where you create an unique index on a new column:"

https://sequelize.org/master/manual/migrations.html#migration-skeleton

[codecov]

Codecov Report

Merging #11759 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #11759   +/-   ##
=======================================
  Coverage   96.26%   96.26%           
=======================================
  Files          94       94           
  Lines        9191     9191           
=======================================
  Hits         8848     8848           
  Misses        343      343

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[papb]

Hello! I see you are a first-time contributor, thank you for taking the time to help Sequelize! I hope to see more PRs from you in the future!

[papb]

Thanks 🎉