fix: Remove days from expiry option value examples

[nning]

The -e option for preauthkeys create and apikeys create is processed with time.ParseDuration which does not support the d suffix (for days) and also no other "bigger" suffixes:

• https://pkg.go.dev/time#ParseDuration
• flag: let Duration accept days golang/go#11473

First I thought I could fix the CLI code but as it's just using cobra to read the expiration flag, I changed the documentation for the commands accepting the -e flag.