Add comment about which method is used to approximate pi

approximate-pi.c

@@ -6,6 +6,7 @@ double approximate_pi(unsigned int steps) {
   double approximate_pi = 4.0;
   bool subtract = true;
 
+  /* Gregory–Leibniz series */
   for (unsigned int i = 2; i <= steps; i++) {
     if (subtract) {
       approximate_pi -= (4.0 / (i * 2 - 1));