function formatIssue( mention ) {
	return '[#' + mention.ref + '](' + mention.url + ')';
}
Formats an issue.

@private
@param {Object} mention - mention object
@returns {string} formatted issue

@example
var out = formatIssue({
    'ref': '123',
    'url': 'https://github.com/stdlib-js/stdlib/issues/123'
});
// returns '[#123](https://github.com/stdlib-js/stdlib/issues/123)'