doc: Fix incorrect sendmany RPC doc

This enables the type check and fixes the wrong docs.

Otherwise the enabled check would lead to test errors, such as:

> "wallet_labels.py", line 96, in run_test
>     node.sendmany(
>
> test_framework.authproxy.JSONRPCException:
>  JSON value of type null is not of expected type string (-3)

src/wallet/rpc/spend.cpp

@@ -314,12 +314,11 @@ RPCHelpMan sendtoaddress()
 RPCHelpMan sendmany()
 {
     return RPCHelpMan{"sendmany",
-                "\nSend multiple times. Amounts are double-precision floating point numbers." +
+        "Send multiple times. Amounts are double-precision floating point numbers." +
         HELP_REQUIRING_PASSPHRASE,
                 {
-                    {"dummy", RPCArg::Type::STR, RPCArg::Optional::NO, "Must be set to \"\" for backwards compatibility.",
+                    {"dummy", RPCArg::Type::STR, RPCArg::Default{"\"\""}, "Must be set to \"\" for backwards compatibility.",
                      RPCArgOptions{
-                         .skip_type_check = true,
                          .oneline_description = "\"\"",
                      }},
                     {"amounts", RPCArg::Type::OBJ_USER_KEYS, RPCArg::Optional::NO, "The addresses and amounts",