Fix a typo in extra credit options
Fix a typo in extra credit options

I misspell occurrences more often than I spell it correctly. 
This time I was at least 2 for 3. This commit puts me at 
3 for 3, and fixes #10.
lidhigens/peter-piper,Moser2010/peter-piper,sdtorres2/peter-piper,mkond2015/peter-piper,keawade/peter-piper,calebwr/peter-piper,halfwayawake/peter-piper,mrdcs92/peter-piper,barberboy/peter-piper,unioncollege-webtech/peter-piper