Typo Update Private-Txns-Migration.md

[onasilos]

PR description

I found a typo in the following part:

A critical issue for privacy users with private transactions created using Hyperledger Besu v1.3.4 or earlier has been identified. If you have a network with private transaction created using v1.3.4 or earlier, please read the following and take the appropriate steps:

The phrase "private transaction created" should be "private transactions created".

The corrected sentence:

A critical issue for privacy users with private transactions created using Hyperledger Besu v1.3.4 or earlier has been identified. If you have a network with private transactions created using v1.3.4 or earlier, please read the following and take the appropriate steps:

Thanks for sending a pull request! Have you done the following?

• Checked out our contribution guidelines?
• Considered documentation and added the doc-change-required label to this PR if updates are required.
• Considered the changelog and included an update if required.
• For database changes (e.g. KeyValueSegmentIdentifier) considered compatibility and performed forwards and backwards compatibility tests

Locally, you can run these tests to catch failures early:

• unit tests: ./gradlew build
• acceptance tests: ./gradlew acceptanceTest
• integration tests: ./gradlew integrationTest
• reference tests: ./gradlew ethereum:referenceTests:referenceTests