📝 Fix a config comment

Marlin/Configuration_adv.h

     // Longer prime to clean out a SINGLENOZZLE
     #define TOOLCHANGE_FS_EXTRA_PRIME          0  // (mm) Extra priming length
     #define TOOLCHANGE_FS_PRIME_SPEED    (4.6*60) // (mm/min) Extra priming feedrate
-    #define TOOLCHANGE_FS_WIPE_RETRACT         0  // (mm/min) Retract before cooling for less stringing, better wipe, etc.
+    #define TOOLCHANGE_FS_WIPE_RETRACT         0  // (mm) Retract before cooling for less stringing, better wipe, etc.
 
     // Cool after prime to reduce stringing
     #define TOOLCHANGE_FS_FAN                 -1  // Fan index or -1 to skip