Mistake in reserve_time explanation in parser filter plugin

### Describe the bug

I believe there is a mistake in the explanation for `reserve_time` property in `parser` filter plugin.

It is saying that when `reserve_time: true`, the time of the event is parsed from the specified parsing field (in that case, it is `log`).
However, it is the other way around (i.e. time of the event is kept and parsed time is discarded).

### Link to the problematic documentation

https://docs.fluentd.org/filter/parser#reserve_time

### Expected explanation

When `reserve_time: true`, the output is:

```
time:
2021-06-24 14:33:35.475115751 +0900 (It vary on parsed timestamp)

record:
{
"user": 1
}
```

When `reserve_time: false` (default value), the output is:

```
time:
2021-06-01 00:00:00.000000000 +0900

record:
{
"user": 1
}
```

### Additional context

_No response_