Trivial spelling correction

Marlin/stepper.cpp

 //===========================================================================
 //=============================private variables ============================
 //===========================================================================
-//static makes it inpossible to be called from outside of this file by extern.!
+//static makes it impossible to be called from outside of this file by extern.!
 
 // Variables used by The Stepper Driver Interrupt
 static unsigned char out_bits;        // The next stepping-bits to be output