locking/lockdep,doc: Improve readability of the block matrix

The block condition matrix is using 'E' as the writer notation,
however, the writer reminder below the matrix is using 'W', to make
them consistent and make the matrix more readable, we'd better to use
'W' to represent writer.

Suggested-by: Waiman Long <[e-mail]>
Suggested-by: Boqun Feng <[e-mail]>
Signed-off-by: Xiongwei Song <[e-mail]>
Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Reviewed-by: Boqun Feng <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]

Documentation/locking/lockdep-design.rst

···
 Block condition matrix, Y means the row blocks the column, and N means otherwise.
 
 	+---+---+---+---+
-	|   | E | r | R |
+	|   | W | r | R |
 	+---+---+---+---+
-	| E | Y | Y | Y |
+	| W | Y | Y | Y |
 	+---+---+---+---+
 	| r | Y | Y | N |
 	+---+---+---+---+