Update Lambda.lagda.md Diamond and confluence example

The diamond property written in Agda demand a single step from L->(M|N)? 
Therefore the verbose text should reflect the picture, which has 3 lines at the top.

Author: b-m-f

src/plfa/part2/Lambda.lagda.md

@@ -793,8 +793,8 @@ It can be illustrated as follows:
 Here `L`, `M`, `N` are universally quantified while `P`
 is existentially quantified.  If each line stands for zero
 or more reduction steps, this is called confluence,
-while if the top two lines stand for a single reduction
-step and the bottom two stand for zero or more reduction
+while if the top three lines stand for a single reduction
+step and the bottom three stand for zero or more reduction
 steps it is called the diamond property. In symbols:
 
 ```agda