Update timestamp error message. (#4099)

## Motivation

The error message refers to a `Tick` operation, which doesn't exist
anymore.

## Proposal

Update the message.

## Test Plan

CI

## Release Plan

- Nothing to do / These changes follow the usual release cycle.

## Links

- [reviewer
checklist](https://github.com/linera-io/linera-protocol/blob/main/CONTRIBUTING.md#reviewer-checklist)

Author: afck

linera-core/src/worker.rs

@@ -193,7 +193,7 @@ pub enum WorkerError {
         computed: Box<BlockExecutionOutcome>,
         submitted: Box<BlockExecutionOutcome>,
     },
-    #[error("The timestamp of a Tick operation is in the future.")]
+    #[error("The block timestamp is in the future.")]
     InvalidTimestamp,
     #[error("We don't have the value for the certificate.")]
     MissingCertificateValue,