Documentation: Refer to git-commit-tree in git-filter-branch help

Currently, the help for git filter-branch refers users of --env-filter
to git-commit for information about environment variables affecting
commits. However, this information is not contained in the git-commit
help, but is very explicitly detailed in git-commit-tree.

Signed-off-by: Wesley J. Landaker <wjl@icecavern.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-filter-branch.txt

@@ -81,7 +81,7 @@ OPTIONS
 	This filter may be used if you only need to modify the environment
 	in which the commit will be performed.  Specifically, you might
 	want to rewrite the author/committer name/email/time environment
-	variables (see linkgit:git-commit[1] for details).  Do not forget
+	variables (see linkgit:git-commit-tree[1] for details).  Do not forget
 	to re-export the variables.
 
 --tree-filter <command>::