documentation: fix alphabetic ordered list for git-rebase man page

An alphabetic ordered list (a.) is converted to numerical in
the man page (1.) so context comments naming 'a' were confusing,
fix that by not using ordered list notation for 'a' anb 'b' items.

Signed-off-by: Nelson Benitez Leon <nelsonjesus.benitez@seap.minhap.es>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rebase.txt

@@ -409,10 +409,13 @@ The interactive mode is meant for this type of workflow:
 
 where point 2. consists of several instances of
 
-a. regular use
+a) regular use
+
  1. finish something worthy of a commit
  2. commit
-b. independent fixup
+
+b) independent fixup
+
  1. realize that something does not work
  2. fix that
  3. commit it