message: schema: fix failing doc test
diff: @@ -61,7 +61,7 @@ pub use cosmwasm_schema_derive::generate_api;
  /// #[derive(JsonSchema)]
  /// struct MigrateMsg;
  ///
  -/// generate_api! {
  +/// write_api! {
  /// name: "cw20",
  /// instantiate: InstantiateMsg,
  /// migrate: MigrateMsg,
  