Quote cols attribute in Asciidoc table example

Closes gh-482

Author: bartolom

docs/src/docs/asciidoc/working-with-asciidoctor.adoc

@@ -136,7 +136,7 @@ example, the widths of a table's columns can be specified using the `cols` attri
 
 [source,indent=0]
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-[cols=1,3] <1>
+[cols="1,3"] <1>
 \include::{snippets}/index/links.adoc[]
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 <1> The table's width will be split across its two columns with the second column being