docs: fix minor glitch in illustration

In the example by Jon Loeliger the selector 'A^2' was duplicated. This
might confuse readers.

Signed-off-by: Michael F. Schönitzer <michael@schoenitzer.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/revisions.txt

@@ -233,7 +233,7 @@ G   H   I   J
 
     A =      = A^0
     B = A^   = A^1     = A~1
-    C = A^2  = A^2
+    C =      = A^2
     D = A^^  = A^1^1   = A~2
     E = B^2  = A^^2
     F = B^3  = A^^3