Added "filetype=asciidoc" to help file modlines.

At the moment, only "syntax=asciidoc" was specified. The
asciidoc_filetype.vim that is distributed with Asciidoc gives the
installer two options -- to either associate every *.txt file with
asciidoc or to try to decipher whether or not the file is an asciidoc
using the first 50 lines.

So it's not given that a developer will open the *.txt files with
ft=asciidoc, and so it's helpful to have it explicitly in the modeline.

vimperator/locale/en-US/options.txt

@@ -826,4 +826,4 @@ A regexp which defines the word separators which are used for the
 the text of a link.
 ____
 
-// vim: set syntax=asciidoc:
+// vim: set syntax=asciidoc filetype=asciidoc: