Update GRADES.md

Under the instructions to use the `scripts/calcgrade.sh` script to check our overall grade, I changed: "The `calcgrade.sh` script expects a single argument that corresponds to the github account of the grade you want to check." to "The `calcgrade.sh` script expects a single argument that corresponds to the github account of the grade you want to check." 

I thought this would be clearer since appending my github account didn't work for me but my ucr netid/cs account did, and based on what the instructors said in issue #303.