Replace ftostr62sign with ftostr62rj

`ftostr62sign()` is used only when displaing/editing
Steps/mm. A sign is not needed - the value is always positive.
Because the number part is long there is no't much place for the values name.
With this PR the is one more char for the name possible.

Marlin/utility.h

@@ -63,8 +63,8 @@ void safe_delay(millis_t ms);
   // Convert signed float to string with +123.45 format
   char* ftostr52sign(const float& x);
 
-  // Convert signed float to string with +1234.56 format
-  char* ftostr62sign(const float& x);
+  // Convert unsigned float to string with 1234.56 format omitting trailing zeros
+  char* ftostr62rj(const float& x);
 
   // Convert float to rj string with 123 or -12 format
   FORCE_INLINE char *ftostr3(const float& x) { return itostr3((int)x); }