Update CONTRIBUTING.md
Update CONTRIBUTING.md

Spelling fix - removed unnecessary "by"

# Before 

```
# Contributing to Famo.us

Active involvement from the community is essential to help make famous the most
capable and performant front-end JavaScript framework out there. You can help by
reporting bugs, fixing bugs, adding features, contributing new modules and by
by providing feedback.
```

# After
```
# Contributing to Famo.us

Active involvement from the community is essential to help make famous the most
capable and performant front-end JavaScript framework out there. You can help by
reporting bugs, fixing bugs, adding features, contributing new modules and by
providing feedback.
```

trusktr/famous,SuPenguin/famous,Faiz7412/famous,ziteboard/famous,SeanOceanHu/famous,wgester/famous,chinakids/famous,gdi2290/famous,Faiz7412/famous,Offirmo/famous,holmesal/famous-global-build,Bizboard/famous,Lkraljevic/famous,Famous/famous,ziteboard/famous,Famous/famous