Use US spelling of 'behavior'

Marlin/src/gcode/bedlevel/G26.cpp

@@ -87,7 +87,7 @@
  *   D    Disable     Disable the Unified Bed Leveling System. In the normal case the user is invoking this
  *                    command to see how well a Mesh as been adjusted to match a print surface. In order to do
  *                    this the Unified Bed Leveling System is turned on by the G26 command. The D parameter
- *                    alters the command's normal behaviour and disables the Unified Bed Leveling System even if
+ *                    alters the command's normal behavior and disables the Unified Bed Leveling System even if
  *                    it is on.
  *
  *   H #  Hotend      Set the Nozzle Temperature. If not specified, a default of 205 C. will be assumed.
@@ -131,7 +131,7 @@
  *   U #  Random      Randomize the order that the circles are drawn on the bed. The search for the closest
  *                    un-drawn circle is still done. But the distance to the location for each circle has a
  *                    random number of the specified size added to it. Specifying S50 will give an interesting
- *                    deviation from the normal behaviour on a 10 x 10 Mesh.
+ *                    deviation from the normal behavior on a 10 x 10 Mesh.
  *
  *   X #  X Coord.    Specify the starting location of the drawing activity.
  *

Marlin/src/gcode/config/M301.cpp

@@ -42,7 +42,7 @@
 void GcodeSuite::M301() {
 
   // multi-extruder PID patch: M301 updates or prints a single extruder's PID values
-  // default behaviour (omitting E parameter) is to update for extruder 0 only
+  // default behavior (omitting E parameter) is to update for extruder 0 only
   const uint8_t e = parser.byteval('E'); // extruder being updated
 
   if (e < HOTENDS) { // catch bad input value

Marlin/src/gcode/control/M999.cpp

@@ -29,7 +29,7 @@
 /**
  * M999: Restart after being stopped
  *
- * Default behaviour is to flush the serial buffer and request
+ * Default behavior is to flush the serial buffer and request
  * a resend to the host starting on the last N line received.
  *
  * Sending "M999 S1" will resume printing without flushing the