Remove redundant [] in backupPhrase doc example
Remove redundant [] in backupPhrase doc example

Mirroring input-output-hk/cardano-wallet#263

input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype