doc: clarify wording on section 3.1

StephenM347 on #lightning-dev indicated it could be clearer that
this is all about changing commitment transactions.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

doc/deployable-lightning.lyx

@@ -584,7 +584,7 @@ Poon-Dryja Generalized Payment Channel Modifications
 \end_layout
 
 \begin_layout Standard
-This paper proposes three of these.
+This paper proposes three changes to the commitment transactions.
 \end_layout
 
 \begin_layout Subsubsection