Revert "filter-branch docs: remove brackets so not to imply revision arg is optional"

This reverts commit c41b439244, as
we decided to default to HEAD when revision parameters are missing
and they are no longer mandatory.

Documentation/git-filter-branch.txt

@@ -13,7 +13,7 @@ SYNOPSIS
 	[--msg-filter <command>] [--commit-filter <command>]
 	[--tag-name-filter <command>] [--subdirectory-filter <directory>]
 	[--original <namespace>] [-d <directory>] [-f | --force]
-	<rev-list options>
+	[<rev-list options>...]
 
 DESCRIPTION
 -----------

git-filter-branch.sh

@@ -92,7 +92,7 @@ USAGE="[--env-filter <command>] [--tree-filter <command>] \
 [--msg-filter <command>] [--commit-filter <command>] \
 [--tag-name-filter <command>] [--subdirectory-filter <directory>] \
 [--original <namespace>] [-d <directory>] [-f | --force] \
-<rev-list options>"
+[<rev-list options>...]"
 
 OPTIONS_SPEC=
 . git-sh-setup