Update constants.md

fix syntax

Author: ReneWerner87

docs/api/constants.md

@@ -47,7 +47,8 @@ const (
     MIMEApplicationXMLCharsetUTF8        = "application/xml; charset=utf-8"
     MIMEApplicationJSONCharsetUTF8       = "application/json; charset=utf-8"
     MIMEApplicationJavaScriptCharsetUTF8 = "application/javascript; charset=utf-8"
-)```
+)
+```
 
 ### HTTP status codes were copied from net/http.