add link for iso-8601 fixes #2

Author: russbiggs

src/content/docs/using-the-api/dates-datetimes.mdx

@@ -14,7 +14,7 @@ import { Icon } from '@astrojs/starlight/components';
 
 ### Datetimes
 
-Dates and datetimes in the OpenAQ API follow the <a href="/"
+Dates and datetimes in the OpenAQ API follow the <a href="https://en.wikipedia.org/wiki/ISO_8601"
 target="_blank" rel="noopener noreferrer" style="display: inline-flex;">ISO-8601<Icon name="external"/></a> datetime
 format, e.g., `YYYY-MM-DDTHH:MM:SS.SSSSZ`.