git-rebase.txt: fix typo

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rebase.txt

@@ -605,7 +605,7 @@ BEHAVIORAL DIFFERENCES
 -----------------------
 
 git rebase has two primary backends: apply and merge.  (The apply
-backend used to known as the 'am' backend, but the name led to
+backend used to be known as the 'am' backend, but the name led to
 confusion as it looks like a verb instead of a noun.  Also, the merge
 backend used to be known as the interactive backend, but it is now
 used for non-interactive cases as well.  Both were renamed based on