filter-branch: add --prune-empty to option summary

Signed-off-by: NAdam Brewster <adambrewster@gmail.com>
Signed-off-by: NJeff King <peff@peff.net>

Documentation/git-filter-branch.txt

@@ -12,6 +12,7 @@ SYNOPSIS
 	[--index-filter <command>] [--parent-filter <command>]
 	[--msg-filter <command>] [--commit-filter <command>]
 	[--tag-name-filter <command>] [--subdirectory-filter <directory>]
+	[--prune-empty]
 	[--original <namespace>] [-d <directory>] [-f | --force]
 	[--] [<rev-list options>...]