Fix typo: explicty -> explicitly in server/service/packs.go (#45874)

This is an independent contribution. No part of this PR was generated,
reviewed, or influenced by any competitive or automated system.

## Summary
Fix a typo in a code comment: `explicty` → `explicitly`

## Root Cause
The comment on `TotalHostsCount` in `server/service/packs.go` contains
the misspelling `explicty` instead of `explicitly`.

## Fix
Corrected the spelling in the comment at line 18.

## Testing
- No functional changes — comment-only fix
- Go compilation unaffected

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## Summary by CodeRabbit

* **Chores**
* Corrected spelling errors in internal code comments to improve code
quality.

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server/service/packs.go

@@ -15,7 +15,7 @@ type packResponse struct {
 	fleet.Pack
 	QueryCount uint `json:"query_count" renameto:"report_count"`
 
-	// All current hosts in the pack. Hosts which are selected explicty and
+	// All current hosts in the pack. Hosts which are selected explicitly and
 	// hosts which are part of a label.
 	TotalHostsCount uint `json:"total_hosts_count"`