fix: lowercase error string in rename processor (ST1005)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: strawgate

libbeat/processors/actions/rename.go

@@ -86,7 +86,7 @@ func (f *renameFields) Run(event *beat.Event) (*beat.Event, error) {
 	for _, field := range f.config.Fields {
 		err := f.renameField(field.From, field.To, event)
 		if err != nil {
-			errMsg := fmt.Errorf("Failed to rename fields in processor: %w", err)
+			errMsg := fmt.Errorf("failed to rename fields in processor: %w", err)
 			f.logger.Debugw(errMsg.Error(), logp.TypeKey, logp.EventType)
 
 			if f.config.FailOnError {