Python: Small docstring fix (microsoft#5741)

### Motivation and Context

This is a single-line fix to a Python docstring to get it to conform to
Sphinx standards and convention used
[elsewhere](https://github.com/microsoft/semantic-kernel/blob/20304f38fed9730f258acbdf59bad898ac809952/python/semantic_kernel/planners/plan.py#L137)
in the project.

(My team owns the reference docs generation process for this and other
libraries. We forced a build the other day from our side for test
purposes and noticed it stumbled over this line. Feel free to ping me if
any questions.)

### Description

See changes, it's a one-liner 😁

### Contribution Checklist

I ashamedly admit I have done *none* of the below; hopefully this is a
gimme, apologies if it creates noise.

- [ ] The code builds clean without any errors or warnings
- [ ] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [ ] All unit tests pass, and I have added new tests where possible
- [ ] I didn't break anyone 😄

python/semantic_kernel/events/function_invoked_event_args.py

@@ -24,7 +24,7 @@ class FunctionInvokedEventArgs(KernelEventArgs):
         kernel_function_metadata (KernelFunctionMetadata): The function that is being executed.
         arguments (KernelArguments): The arguments that are being passed to the function.
         function_result (FunctionResult): The result of the function execution.
-        exception (Optional: Exception): The exception that was raised during the function execution.
+        exception (Exception, optional): The exception that was raised during the function execution.
 
     Flags:
         updated_arguments (bool): Whether the arguments were updated, default False.