Fine-tune grammar. (#108)

My "OCD" kicked in. I believe "open source" is grammatically correct, so let us therefore use this rather than "opensource". It looks better too :)

README.md

@@ -4,7 +4,7 @@
 [![Code Climate](https://codeclimate.com/github/elrido/ZeroBin/badges/gpa.svg)](https://codeclimate.com/github/elrido/ZeroBin)
 [![Test Coverage](https://codeclimate.com/github/elrido/ZeroBin/badges/coverage.svg)](https://codeclimate.com/github/elrido/ZeroBin/coverage)
 
-ZeroBin is a minimalist, opensource online pastebin where the server has zero 
+ZeroBin is a minimalist, open source online pastebin where the server has zero 
 knowledge of pasted data.
 
 Data is encrypted/decrypted in the browser using 256 bit AES.