message: fix testmempoolaccept CLI syntax
  `testmempoolaccept "hexstring"` will give a "JSON parse error". The correct syntax is `testmempoolaccept \[\"hexstring\"\]` (but seems escaping is not displayed in other areas so leaving backspaces out).
diff: @@ -1135,7 +1135,7 @@ static UniValue testmempoolaccept(const JSONRPCRequest& request)
  "Sign the transaction, and get back the hex\n"
  + HelpExampleCli("signrawtransactionwithwallet", "\"myhex\"") +
  "\nTest acceptance of the transaction (signed hex)\n"
  - + HelpExampleCli("testmempoolaccept", "\"signedhex\"") +
  + + HelpExampleCli("testmempoolaccept", "[\"signedhex\"]") +
  "\nAs a JSON-RPC call\n"
  + HelpExampleRpc("testmempoolaccept", "[\"signedhex\"]")
  );
  