Few minor fixes for the grade-school exercise

Looking at the nitpicks for this problem, a lot of people were thrown by the `grade/2` test mentioning alphabetical order. The test was not written to actually test for alphabetical order, and the README did not indicate that it would be required...so, I just updated the description for that test.

I also fixed a compiler warning on the spec definition for `add/3` and converted everything to use spaces instead of tabs (per community standards, and to match all other exercises thus far).