Docs: correct 'OnBufferedAmountLow' condition

This is in line with
* the docs for 'BufferedAmountLowThreshold' ("When the bufferedAmount
decreases from above this threshold to equal or below it")

* The actual code in 'pion/sctp':
https://github.com/pion/sctp/blob/d6446e3b0d0495d9974e68fec9b7fdb79c018ca1/stream.go#L422

* The Web WebRTC spec:
https://w3c.github.io/webrtc-pc/#event-datachannel-bufferedamountlow

Author: WofWca

datachannel.go

@@ -578,7 +578,7 @@ func (d *DataChannel) SetBufferedAmountLowThreshold(th uint64) {
 }
 
 // OnBufferedAmountLow sets an event handler which is invoked when
-// the number of bytes of outgoing data becomes lower than the
+// the number of bytes of outgoing data becomes lower than or equal to the
 // BufferedAmountLowThreshold.
 func (d *DataChannel) OnBufferedAmountLow(f func()) {
 	d.mu.Lock()

datachannel_js.go

@@ -246,7 +246,7 @@ func (d *DataChannel) SetBufferedAmountLowThreshold(th uint64) {
 }
 
 // OnBufferedAmountLow sets an event handler which is invoked when
-// the number of bytes of outgoing data becomes lower than the
+// the number of bytes of outgoing data becomes lower than or equal to the
 // BufferedAmountLowThreshold.
 func (d *DataChannel) OnBufferedAmountLow(f func()) {
 	if d.onBufferedAmountLow != nil {