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.
```