Patch steps rate comment in trapezoid function

Marlin/planner.cpp

 // Calculates trapezoid parameters so that the entry- and exit-speed is compensated by the provided factors.
 
 void calculate_trapezoid_for_block(block_t* block, float entry_factor, float exit_factor) {
-  unsigned long initial_rate = ceil(block->nominal_rate * entry_factor); // (step/min)
-  unsigned long final_rate = ceil(block->nominal_rate * exit_factor); // (step/min)
+  unsigned long initial_rate = ceil(block->nominal_rate * entry_factor),
+                final_rate = ceil(block->nominal_rate * exit_factor); // (steps per second)
 
   // Limit minimal step rate (Otherwise the timer will overflow.)
   NOLESS(initial_rate, 120);