git-filter-repo.txt: add missing `git-` prefix to fast-import link

Signed-off-by: Todd Zullinger <tmz@pobox.com>
Signed-off-by: Elijah Newren <newren@gmail.com>

Documentation/git-filter-repo.txt

@@ -1047,7 +1047,7 @@ instead of strings.
 
 There are four callbacks that allow you to operate directly on raw
 objects that contain data that's easy to write in
-linkgit:fast-import[1] format:
+linkgit:git-fast-import[1] format:
 
 --------------------------------------------------
 --blob-callback