Correct DEFAULT_MINSEGMENTTIME units

Marlin/Configuration_adv.h

@@ -770,7 +770,7 @@
 //#define HOME_AFTER_DEACTIVATE  // Require rehoming after steppers are deactivated
 
 // Minimum time that a segment needs to take if the buffer is emptied
-#define DEFAULT_MINSEGMENTTIME        20000   // (ms)
+#define DEFAULT_MINSEGMENTTIME        20000   // (µs)
 
 // Slow down the machine if the look ahead buffer is (by default) half full.
 // Increase the slowdown divisor for larger buffer sizes.