Fix grammar on home page by changing , to ;

Affects this section of the [home page](https://gratipay.com/):

![section of home page with grammar error](https://cloud.githubusercontent.com/assets/79168/4230969/f9a475b0-398a-11e4-913f-0ff97180dc87.png)

I double-checked correctness of the change against the grammar rules in “Purdue Online Writing Lab”’s [Commas vs. Semicolons in Compound Sentences](https://owl.english.purdue.edu/owl/resource/607/04/).

*keywords:* comma to semicolon