doc: fix error in fast-escape redeemscript.

Reported by killerstorm on reddit:
https://www.reddit.com/r/Bitcoin/comments/3dlxw4/reaching_the_ground_with_lightning_lightning/ct6jyim

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

doc/deployable-lightning.lyx

@@ -1454,7 +1454,7 @@ OP_HASH
 \begin_inset space ~
 \end_inset
 
-<SECRET-B>
+<SECRET-B-HASH>
 \begin_inset space ~
 \end_inset