doc: format-patch: fix typo

reroll count documentation states that v<n> will be pretended to the
filename. Judging by the examples that should have been 'prepended'.

Signed-off-by: Frans Klaver <fransklaver@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-format-patch.txt

@@ -168,7 +168,7 @@ will want to ensure that threading is disabled for `git send-email`.
 
 --reroll-count=<n>::
 	Mark the series as the <n>-th iteration of the topic. The
-	output filenames have `v<n>` pretended to them, and the
+	output filenames have `v<n>` prepended to them, and the
 	subject prefix ("PATCH" by default, but configurable via the
 	`--subject-prefix` option) has ` v<n>` appended to it.  E.g.
 	`--reroll-count=4` may produce `v4-0001-add-makefile.patch`