preheat: improve target temperature threshold

Use 5°C threshold to be consistant with other parts
of the firmware.

Relying on 95% of the target temperature creates
a dependency on the temperature:

PLA: Target = 215°C, threshold = 10.75°C
PETG: Target = 230°C, threshold = 11.5°C
ABS: Target = 255°C, threshold  = 12.75°C
ASA: Target =260°C, threshold = 13.0°C
PC: Target = 275°C, threshold  = 13.75°C

My proposal is we instead use a constant
TEMP_HYSTERESIS = 5, which is consistent with
M109, and behavior when restoring print from RAM
and some of the MMU code (like unload function)

Change in memory:
Flash: +2 bytes
SRAM: 0 bytes

Firmware/ultralcd.cpp

@@ -1888,7 +1888,7 @@ void mFilamentItem(uint16_t nTemp, uint16_t nTempBed)
 
     lcd_timeoutToStatus.stop();
 
-    if (current_temperature[0] > (target_temperature[0] * 0.95))
+    if (fabs(current_temperature[0] - target_temperature[0]) > TEMP_HYSTERESIS)
     {
         switch (eFilamentAction)
         {