custom logger docs link out of date

[nediamond]

The docs link in custom-logger/README.md is outdated: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