Remove redundant [] in backupPhrase doc example

[Anviking]

Description

Mirroring cardano-foundation/cardano-wallet#263

Linked issue

cardano-foundation/cardano-wallet#263

Type of change

• 🐞 Bug fix (non-breaking change which fixes an issue)
• 🛠 New feature (non-breaking change which adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)
• 🏭 Refactoring that does not change existing functionality but does improve things like code readability, structure etc
• 🔨 New or improved tests for existing code
• ⛑ git-flow chore (backport, hotfix, etc)

Developer checklist

• I have read the style guide document, and my code follows the code style of this project.
• If my code deals with exceptions, it follows the guidelines.
• I have updated any documentation accordingly, if needed. Documentation changes can be reflected in opening a PR on cardanodocs.com, amending the inline Haddock comments, any relevant README file or one of the document listed in the docs directory.
• CHANGELOG entry has been added and is linked to the correct PR on GitHub.

Testing checklist

• I have added tests to cover my changes.
• All new and existing tests passed.

QA Steps

Screenshots (if available)

How to merge

Send the message bors r+ to merge this PR. For more information, see
docs/how-to/bors.md.

[KtorZ]

bors r+

[iohk-bors]

Build succeeded

• ci/hydra:serokell:cardano-sl:required
• buildkite/cardano-sl