custom logger docs link out of date

The docs link in `custom-logger/README.md` is outdated:

Currently is:
```
The [Logging docs](https://docs.temporal.io/dev-guide/typescript/observability#logging) explain some of the code in this sample.
```

Should be:
```
The [Logging docs](https://docs.temporal.io/develop/typescript/observability#logging) explain some of the code in this sample.
```