Rename Ssrc -> SSRC

This capitalization is consistent with other uses in the project,
and matches the Go convention of capitalizing initialisms.

https://github.com/golang/go/wiki/CodeReviewComments#initialisms

Relates to #417

examples/save-to-disk/main.go

@@ -41,7 +41,7 @@ func main() {
 		go func() {
 			ticker := time.NewTicker(time.Second * 3)
 			for range ticker.C {
-				err := peerConnection.SendRTCP(&rtcp.PictureLossIndication{MediaSSRC: track.Ssrc})
+				err := peerConnection.SendRTCP(&rtcp.PictureLossIndication{MediaSSRC: track.SSRC})
 				if err != nil {
 					fmt.Println(err)
 				}