sys/zptr: fix typos
sys/zptr: fix typos

- 'bounderies' -> 'boundaries'
- 'compressable' -> 'compressible'
- 'Substracts' -> 'Subtracts'

OlegHahm/RIOT,ant9000/RIOT,authmillenon/RIOT,miri64/RIOT,OlegHahm/RIOT,kYc0o/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,kaspar030/RIOT,miri64/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,ant9000/RIOT,authmillenon/RIOT,kYc0o/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,jasonatran/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,miri64/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,jasonatran/RIOT,kaspar030/RIOT,authmillenon/RIOT,OTAkeys/RIOT,authmillenon/RIOT,miri64/RIOT,jasonatran/RIOT,OTAkeys/RIOT,miri64/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,jasonatran/RIOT,ant9000/RIOT,kYc0o/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,ant9000/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,authmillenon/RIOT,ant9000/RIOT,kaspar030/RIOT