Update concepts.dd

The old version of isprime would return false for 2 and return true for any n with no factor less than sqrt(n).

These alterations fix that.
Update concepts.dd

in function isPrime: reversed the change to the condition in the loop. Changed name to isPrime in keeping with style guide.
Update concepts.dd

in isPrime: fixed braces style and corrected inequality.
Merge pull request #244 from John-Colvin/patch-1

fix the algorithm in the isprime example