docs(theme): fix alert border

Author: Pyrax

www/src/gatsby-plugin-theme-ui/index.js

@@ -98,7 +98,9 @@ export default merge(myTheme, {
       color: 'text',
       bg: 'muted',
       display: 'block',
-      border: (theme) => `2px solid ${theme.colors.highlight}`,
+      borderRadius: '2px',
+      borderStyle: 'solid',
+      borderColor: 'highlight',
       'h1:first-of-type, h2:first-of-type, h3:first-of-type, h4:first-of-type, h5:first-of-type, h6:first-of-type': {
         mt: 2
       }