daml-ledger.ts: Rename create argument to payload (#4198)

This is better in line with the field name of the `CreatedEvent` you'll
receive as a result of calling this.

CHANGELOG_BEGIN
CHANGELOG_END

language-support/ts/daml-ledger/index.ts

@@ -157,10 +157,10 @@ class Ledger {
   /**
    * Create a contract for a given template.
    */
-  async create<T extends object, K>(template: Template<T, K>, contractPayload: T): Promise<CreateEvent<T, K>> {
+  async create<T extends object, K>(template: Template<T, K>, payload: T): Promise<CreateEvent<T, K>> {
     const command = {
       templateId: template.templateId,
-      payload: contractPayload,
+      payload,
     };
     const json = await this.submit('command/create', command);
     return jtv.Result.withException(decodeCreateEvent(template).run(json));