Fix PASERK string lengths in README

[francislavoie]

The string lengths in the README don't seem correct.

Examples from the spec:

> strlen("k4.public.cHFyc3R1dnd4eXp7fH1-f4CBgoOEhYaHiImKi4yNjo8")
= 53

> strlen("k4.secret.cHFyc3R1dnd4eXp7fH1-f4CBgoOEhYaHiImKi4yNjo8c5WpIyC_5kWKhS8VEYSZ05dYfuTF-ZdQFV4D9vLTcNQ")
= 96

Reference:

• https://github.com/paseto-standard/paserk/blob/master/types/secret.md
• https://github.com/paseto-standard/paserk/blob/master/types/public.md

The Uint8Array byte sizes should probably be adjusted too, but I'm not sure how to verify this.

[miunau]

Thanks!