runtime(help): fix typo s/additional/arbitrary/

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/helphelp.txt b/runtime/doc/helphelp.txt
index 5ab112d..9e3e748 100644
--- a/runtime/doc/helphelp.txt
+++ b/runtime/doc/helphelp.txt
@@ -461,7 +461,7 @@
 key, the Vim syntax highlighting will not be enabled. If you set it to an
 empty value, syntax highlighting for embedded languages will be disabled.
 
-Further note: including additional syntax languages into help files may not
+Further note: including arbitrary syntax languages into help files may not
 always work perfectly, if the included 'syntax' script does not account for
 such an import.
 						*help-notation*