refs #7401 - fix markdown syntax in API doc

<pre>
$ rake apipie:cache
 ___________________________________________________________________________
| Maruku tells you:
+---------------------------------------------------------------------------
| Could not find ref_id = "eth1, eth2" for md_link([
| 	md_entity("lsquo"),
| 	"eth1",
| 	md_entity("rsquo"),
| 	", ",
| 	md_entity("lsquo"),
| 	"eth2",
| 	md_entity("rsquo")
| ], nil)
| Available refs are []
+---------------------------------------------------------------------------
!/home/dcleal/.rvm/gems/ruby-2.0.0-p353@foreman/gems/maruku-0.7.2/lib/maruku/output/to_html.rb:656:in `to_html_link'
!/home/dcleal/.rvm/gems/ruby-2.0.0-p353@foreman/gems/maruku-0.7.2/lib/maruku/output/to_html.rb:891:in `block in array_to_html'
!/home/dcleal/.rvm/gems/ruby-2.0.0-p353@foreman/gems/maruku-0.7.2/lib/maruku/output/to_html.rb:879:in `each'
!/home/dcleal/.rvm/gems/ruby-2.0.0-p353@foreman/gems/maruku-0.7.2/lib/maruku/output/to_html.rb:879:in `array_to_html'
\___________________________________________________________________________

Not creating a link for ref_id = "eth1, eth2".
</pre>