Don't replace underscores in auto-generated IDs in goldmark (#12805)

Fix #12196

Signed-off-by: Andrew Thornton <art27@cantab.net>

modules/markup/common/footnote.go

@@ -34,7 +34,7 @@ func CleanValue(value []byte) []byte {
 	needsDash := false
 	for _, r := range rs {
 		switch {
-		case unicode.IsLetter(r) || unicode.IsNumber(r):
+		case unicode.IsLetter(r) || unicode.IsNumber(r) || r == '_':
 			if needsDash && len(result) > 0 {
 				result = append(result, '-')
 			}