perf: fix perf3 to show total iters instead of / 3

The run-fn-for function was originally name run-fn and was hard-coded
to run for 3 seconds. I forgot to replace the 3 with the max-secs
variable. However, given that some implementations have less than
1 iteration per second, I'm just going to drop that division and
report the iterations per max-seconds so that for the slowest
implementations we have some relative comparison instead of all of
them just reporting 0.

Thanks to Tim Morgan for discovering this bug:
699f0ad23a

perf.mal

@@ -24,4 +24,4 @@
       ;; Warm it up first
       (run-fn-for* fn 1000 0 0)
       ;; Now do the test
-      (/ (run-fn-for* fn (* 1000 max-secs) 0 0) 3))))
+      (run-fn-for* fn (* 1000 max-secs) 0 0))))

tests/perf3.mal

@@ -5,7 +5,7 @@
 
 (def! atm (atom (list 0 1 2 3 4 5 6 7 8 9)))
 
-(println "iters/s:"
+(println "iters over 10 seconds:"
   (run-fn-for
     (fn* []
       (do