message: doc: correct GetDifficulty doc after
diff: @@ -16,8 +16,7 @@ class UniValue;
  static constexpr int NUM_GETBLOCKSTATS_PERCENTILES = 5;
  /**
  - * Get the difficulty of the net wrt to the given block index, or the chain tip if
  - * not provided.
  + * Get the difficulty of the net wrt to the given block index.
  *
  * @return A floating point number that is a multiple of the main net minimum
  * difficulty (4295032833 hashes).
  