remove unnecessary type conversion

diff --git a/goyaml.go b/goyaml.go
index 77e50e6..4bf3c43 100644
--- a/goyaml.go
+++ b/goyaml.go

@@ -197,9 +197,9 @@
 			recommend := tag[:s]
 			for _, c := range tag[s+1:] {
 				switch c {
-				case rune('c'):
+				case 'c':
 					recommend += ",omitempty"
-				case rune('f'):
+				case 'f':
 					recommend += ",flow"
 				default:
 					msg := fmt.Sprintf("Unsupported flag %q in tag %q of type %s", string([]byte{uint8(c)}), tag, st)