You will have to modify the front-end templating for the package (aka dataset) view. You can either add this change to your own copy of CKAN source code, or like Hendrik mentioned and data.gov.uk people have done, create a CKAN extension to modify the templates.
Your example of the feature is implemented for the data.gov.uk site which is open source and available in GitHub. The specific part is in the read_common.html:
<b>API: </b><a href="${h.url_for(controller='api', register='package', action='show', id=c.pkg.name, ver='2')}">
${h.url_for(controller='api', register='package', action='show', id=c.pkg.name, ver='2')}
</a>
The url_for function is from the ckan.lib.helpers module which "is available for templates as 'h'" according to the CKAN docs, so you should be able to use it directly in the template as in the code above.