fixup! Async.Promise: Fix/brush up the help document

Author: rhysd

doc/vital/Async/Promise.txt

@@ -101,11 +101,11 @@ Before explaining the detail of APIs, let's see actual examples.
   let s:Promise = vital#vital#import('Async.Promise')
 
   function! s:read(chan, part) abort
-    let out = ''
+    let out = []
     while ch_status(a:chan, {'part' : a:part}) =~# 'open\|buffered'
-      let out .= ch_read(a:chan, {'part' : a:part}) . "\n"
+      call add(out, ch_read(a:chan, {'part' : a:part}))
     endwhile
-    return out
+    return join(out, "\n")
   endfunction
 
   function! s:sh(...) abort