Remove mention of lt-cred-mech in the sample coturn config. (#4333)

* Remove mention of lt-cred-mech in the sample coturn config.

See https://github.com/coturn/coturn/pull/262

 for more context.
Also clean up some minor formatting issues while I'm here.

* Add changelog.

Signed-off-by: Krithin Sitaram <krithin@gmail.com>