Fix syntax of #+ATTR_HTML

* doc/org-manual.org (Tables in HTML export): Fix syntax of example.

The example in the "Tables in HTML export" section of the manual
uses the old syntax: `border="2"' etc.

Change it to use `:border 2' etc.

doc/org-manual.org

@@ -12817,7 +12817,7 @@ following lines before the table in the Org file:
 #+cindex: @samp{ATTR_HTML}, keyword
 #+begin_example
 ,#+CAPTION: This is a table with lines around and between cells
-,#+ATTR_HTML: border="2" rules="all" frame="border"
+,#+ATTR_HTML: :border 2 :rules all :frame border
 #+end_example
 
 The HTML export back-end preserves column groupings in Org tables (see