changed bench behavior for slow compression levels

programs/zstd.1

@@ -128,7 +128,7 @@ Typical gains range from ~10% (at 64KB) to x5 better (at <1KB).
  benchmark file(s) using compression level #
 .TP
 .B \-i#
- iteration loops [1-9](default : 3), benchmark mode only
+ minimum evaluation time, in seconds (default : 3s), benchmark mode only
 .TP
 .B \-B#
  cut file into independent blocks of size # (default: no block)