Run Nick G's spellchecker github.com/client9/misspell, carefuly fix what it found (#4235)

src/runtime/runtime_atsamd51.go

@@ -307,7 +307,7 @@ func readRTC() uint32 {
 
 // ticks are in microseconds
 // Returns true if the timer completed.
-// Returns false if another interrupt occured which requires an early return to scheduler.
+// Returns false if another interrupt occurred which requires an early return to scheduler.
 func timerSleep(ticks uint32) bool {
 	timerWakeup.Set(0)
 	if ticks < 8 {