REL: fix some occurrences of "HeartDB"

src/doc/specification/Authentication.md

 # Authentication
- Some features of HeartDB are available to registered users only. Making any changes to the data stock via HTTP requires authentication by `username` _plus_ `password`. They are to be send as a HTTP header, while the password is to be hashed by the sha512 algorithm:
+ Some features of CaosDB are available to registered users only. Making any changes to the data stock via HTTP requires authentication by `username` _plus_ `password`. They are to be send as a HTTP header, while the password is to be hashed by the sha512 algorithm:
 
 | `username:` | `$username` | 
 |-------------|-------------|-