fix bug for api, level should be string.

@@ -146,7 +146,7 @@ int SrsStatisticStream::dumps(stringstream& ss)
         ss  << SRS_JFIELD_NAME("video") << SRS_JOBJECT_START
                 << SRS_JFIELD_STR("codec", srs_codec_video2str(vcodec)) << SRS_JFIELD_CONT
                 << SRS_JFIELD_STR("profile", srs_codec_avc_profile2str(avc_profile)) << SRS_JFIELD_CONT
-                << SRS_JFIELD_ORG("level", srs_codec_avc_level2str(avc_level))
+                << SRS_JFIELD_STR("level", srs_codec_avc_level2str(avc_level))
                 << SRS_JOBJECT_END
             << SRS_JFIELD_CONT;
     }