runtime(doc): add missing help tags

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/ft_rust.txt b/runtime/doc/ft_rust.txt
index fb8141c..564f3e7 100644
--- a/runtime/doc/ft_rust.txt
+++ b/runtime/doc/ft_rust.txt
@@ -160,7 +160,7 @@
 	provided) instead of '--write-mode=overwrite'. >
 	    let g:rustfmt_emit_files = 0
 
-
+<
                                                           *g:rust_playpen_url*
 g:rust_playpen_url~
 	Set this option to override the url for the playpen to use: >