docs: replace receiver by sender in last sentence

Fixes issue obonaventure#180

book-2nd/principles/reliability.rst

@@ -782,7 +782,7 @@ Unfortunately, frame losses do not disappear because a reliable protocol uses a
 
 .. index:: cumulative acknowledgements
 
-The simplest sliding window protocol uses the `go-back-n` recovery. Intuitively, `go-back-n` operates as follows. A `go-back-n` receiver is as simple as possible. It only accepts the frames that arrive in-sequence. A `go-back-n` receiver discards any out-of-sequence frame that it receives. When `go-back-n` receives a data frame, it always returns an acknowledgement containing the sequence number of the last in-sequence frame that it has received. This acknowledgement is said to be `cumulative`. When a `go-back-n` receiver sends an acknowledgement for sequence number `x`, it implicitly acknowledges the reception of all frames whose sequence number is earlier than `x`. A key advantage of these cumulative acknowledgements is that it is easy to recover from the loss of an acknowledgement. Consider for example a `go-back-n` receiver that received frames `1`, `2` and `3`. It sent `OK1`, `OK2` and `OK3`. Unfortunately, `OK1` and `OK2` were lost. Thanks to the cumulative acknowledgements, when the receiver receives `OK3`, it knows that all three frames have been correctly received. 
+The simplest sliding window protocol uses the `go-back-n` recovery. Intuitively, `go-back-n` operates as follows. A `go-back-n` receiver is as simple as possible. It only accepts the frames that arrive in-sequence. A `go-back-n` receiver discards any out-of-sequence frame that it receives. When `go-back-n` receives a data frame, it always returns an acknowledgement containing the sequence number of the last in-sequence frame that it has received. This acknowledgement is said to be `cumulative`. When a `go-back-n` receiver sends an acknowledgement for sequence number `x`, it implicitly acknowledges the reception of all frames whose sequence number is earlier than `x`. A key advantage of these cumulative acknowledgements is that it is easy to recover from the loss of an acknowledgement. Consider for example a `go-back-n` receiver that received frames `1`, `2` and `3`. It sent `OK1`, `OK2` and `OK3`. Unfortunately, `OK1` and `OK2` were lost. Thanks to the cumulative acknowledgements, when the sender receives `OK3`, it knows that all three frames have been correctly received. 
 
 The figure below shows the FSM of a simple `go-back-n` receiver. This receiver uses two variables : `lastack` and `next`. `next` is the next expected sequence number and `lastack` the sequence number of the last data frame that has been acknowledged. The receiver only accepts the frame that are received in sequence. `maxseq` is the number of different sequence numbers (:math:`2^n`).