doc: update export filtering example

[yeongjukang]

Fixes #3570

Description

Filter spec has namespace with string list type as below, but document is written using just string, so submitting this PR to fix document.

tetragon/api/v1/tetragon/events.proto

Line 40 in d595559

repeated string namespace = 2;

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[yeongjukang]

@mtardy
How about adding type description column info at the table here?
https://tetragon.io/docs/concepts/events/#list-of-process-event-filters

[mtardy]

@mtardy How about adding type description column info at the table here? https://tetragon.io/docs/concepts/events/#list-of-process-event-filters

sure, if that can help :) it seems almost all of them are array?

[yeongjukang]

@mtardy Thank you for the review!
Yes, most of them are array. My concerns were mobile readability gets bad and difference from proto code.

How about just embedding permlink of the main branch of filter message like this block?

...

##### List of Process Event Filters

- The type for each filter attribute can be determined by referring to the [events.proto](https://github.com/cilium/tetragon/blob/main/api/v1/tetragon/events.proto#L38) file.
| Filter | Description | 
| ------ | ----------- |
| `event_set` | Filter process events by event types. Supported types include: `PROCESS_EXEC`, `PROCESS_EXIT`, `PROCESS_KPROBE`, `PROCESS_UPROBE`, `PROCESS_TRACEPOINT`, `PROCESS_LOADER` |

...

[mtardy]

How about just embedding permlink of the main branch of filter message like this block?

yep that's an idea!

[mtardy]

Let's merge this fix, please make another patch if you want to add a link to the protobuf in the docs

[yeongjukang]

Thank you, I will do it immediately.