Fix typo in primality exercise

Author: rbuch

content/exercises/primality.html

@@ -79,8 +79,8 @@ <h2>Part B (grainsize control)</h2>
 the results have been returned (but before they are printed), and
 printing the difference between the timers.  You may
 omit (and probably should omit) printing primality results for performance runs.
-Vary M and report smallest G for
-which performance was within 5% infinite grainsize (i.e. $G == K$). 
+Vary M and report smallest M for
+which performance was within 5% infinite grainsize (i.e. $M == K$).
 Again, make sure our artificial restriction is obeyed: do not send
 back the numbers the number being tested (because you are not allowed
 to search for it anyway).</p>