fix: correct typo "occured" to "occurred" in ErrorChain comment (#45871)

This is an independent contribution made by an individual developer.
This work is not associated with any hackathon, competition, or
coordinated PR campaign.

## Summary

Fixes a typo in a comment in `server/mdm/nanomdm/mdm/command.go` where
"occured" should be "occurred" in the `ErrorChain` type documentation.

## Root Cause

The comment on line 14 of `command.go` describing the `ErrorChain` type
uses the misspelling "occured" instead of the correct spelling
"occurred".

## Fix

Changed "occured" to "occurred" in the type documentation comment.

## Changes

- `server/mdm/nanomdm/mdm/command.go`: 1 line changed (`+1 -1`)

## Testing

- The change modifies only a comment string; no functional behavior is
affected.
- `go build ./...` passes with no regressions.
- The `ErrorChain` type documentation now uses the correct spelling. ✅

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## Summary by CodeRabbit

* **Chores**
  * Fixed spelling in internal comments.

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server/mdm/nanomdm/mdm/command.go

@@ -11,7 +11,7 @@ var (
 	ErrInvalidCommand       = errors.New("invalid command")
 )
 
-// ErrorChain represents errors that occured on the client executing an MDM command.
+// ErrorChain represents errors that occurred on the client executing an MDM command.
 type ErrorChain struct {
 	ErrorCode            int
 	ErrorDomain          string