doc: typo from roasbeef on reddit.

https://www.reddit.com/r/Bitcoin/comments/3dlxw4/reaching_the_ground_with_lightning_lightning/ct78mk9

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

doc/deployable-lightning.lyx

@@ -755,7 +755,7 @@ This form allows A and B to create commitment transactions which spends
 \end_layout
 
 \begin_layout Standard
-This we introduce an 
+Thus we introduce an 
 \emph on
 escape
 \emph default