Fix typo

Author: rhc2104

README.md

@@ -159,7 +159,7 @@ Contributors should follow the GitHub [fork-and-pull model](https://help.github.
     - `git push origin branch_name`
 3. Go to the code-dot-org GitHub page
     - [https://github.com/code-dot-org/code-dot-org](https://github.com/code-dot-org/code-dot-org)
-4. For your submissinon to be reviewed
+4. For your submission to be reviewed
     - Click on the "Pull Request" link, look over and confirm your diff
     - Submit a pull request for your branch to be merged into staging
     - For bonus points, include screenshots in the description. Command + Ctrl + Shift + 4 in OS X lets you copy a screen selection to your clipboard, which GitHub will let you paste right into the description