Update CONTRIBUTE.md
Update CONTRIBUTE.md

Quick fix a typo.
Rud5G/zf2-documentation,akrabat/zf2-documentation,Rud5G/zf2-documentation,gennadiylitvinyuk/zf2-documentation,rettal/zf2-documentation,samsonasik/zf2-documentation,rettal/zf2-documentation,akrabat/zf2-documentation,gennadiylitvinyuk/zf2-documentation,samsonasik/zf2-documentation