subscriber: update tracing-subscriber version in README (#2273)

`tracing-subscriber` has been on `0.3` for almost a year now.

I naively copied the versions from the README, and when trying to use
the library with the current versions of e.g. `tracing-opentelemetry`, I
ran into the same error message as seen in this issue:
#852

Co-authored-by: Bryan Garza <1396101+bryangarza@users.noreply.github.com>