docs: correct createwallet help (dashpay#5407)

## Issue being fixed or feature implemented
Dash does not have `sethdseed`, but the help mentioned it.

## What was done?
Switched to `upgradetohd`.

## How Has This Been Tested?

## Breaking Changes
N/A

## Checklist:
_Go over all the following points, and put an `x` in all the boxes that
apply._
- [x] I have performed a self-review of my own code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have added or updated relevant unit/integration/functional/e2e
tests
- [ ] I have made corresponding changes to the documentation
- [ ] I have assigned this pull request to a milestone _(for repository
code-owners and collaborators only)_

src/wallet/rpcwallet.cpp

@@ -2864,7 +2864,7 @@ static UniValue createwallet(const JSONRPCRequest& request)
         {
             {"wallet_name", RPCArg::Type::STR, RPCArg::Optional::NO, "The name for the new wallet. If this is a path, the wallet will be created at the path location."},
             {"disable_private_keys", RPCArg::Type::BOOL, /* default */ "false", "Disable the possibility of private keys (only watchonlys are possible in this mode)."},
-            {"blank", RPCArg::Type::BOOL, /* default */ "false", "Create a blank wallet. A blank wallet has no keys or HD seed. One can be set using sethdseed."},
+            {"blank", RPCArg::Type::BOOL, /* default */ "false", "Create a blank wallet. A blank wallet has no keys or HD seed. One can be set using upgradetohd."},
             {"passphrase", RPCArg::Type::STR, RPCArg::Optional::OMITTED, "Encrypt the wallet with this passphrase."},
             {"avoid_reuse", RPCArg::Type::BOOL, /* default */ "false", "Keep track of coin reuse, and treat dirty and clean coins differently with privacy considerations in mind."},
             {"load_on_startup", RPCArg::Type::BOOL, /* default */ "null", "Save wallet name to persistent settings and load on startup. True to add wallet to startup list, false to remove, null to leave unchanged."},