typo Update Private-Txns-Migration.md

[0xwitty]

PR description

In the sentence:

"If you have a network with private transaction created using v1.3.4 or earlier, please read the following and take the appropriate steps."

Correction:

"private transaction" should be "private transactions" to maintain consistency and correct plural form.

Fixed Issue(s)

Corrected.

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• 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