Documentation/git-filter-branch: teach "rm" instead of "update-index --remove"

The example to remove paths using index-filter was done with
"git update-index --remove"; "git rm --cached" would be more familiar to
new people and is sufficient for this particular case.
Signed-off-by: NPetr Baudis <pasky@suse.cz>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-filter-branch.txt

@@ -191,7 +191,7 @@ Thus you may instead want to use `rm -f filename` as the script.
 A significantly faster version:
 
 --------------------------------------------------------------------------
-git filter-branch --index-filter 'git update-index --remove filename' HEAD
+git filter-branch --index-filter 'git rm --cached filename' HEAD
 --------------------------------------------------------------------------
 
 Now, you will get the rewritten history saved in HEAD.