net/rpc: fix method requirement docs

The receiver itself is not transmitted and does not need to be
marshalable by encoding/gob.

Fixes #16803.

Change-Id: I42a3603fb7d3b36c97dcc2e51a398cd65ec3227d
Reviewed-on: https://go-review.googlesource.com/32094
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/net/rpc/server.go

@@ -23,7 +23,7 @@
 
 		func (t *T) MethodName(argType T1, replyType *T2) error
 
-	where T, T1 and T2 can be marshaled by encoding/gob.
+	where T1 and T2 can be marshaled by encoding/gob.
 	These requirements apply even if a different codec is used.
 	(In the future, these requirements may soften for custom codecs.)