Fix API footer link in Web UI (#13441)

Co-authored-by: koyu <me@koyu.spac>

app/views/layouts/public.html.haml

@@ -39,7 +39,7 @@
             %h4= t 'footer.developers'
             %ul
               %li= link_to t('about.documentation'), 'https://docs.joinmastodon.org/'
-              %li= link_to t('about.api'), 'https://docs.joinmastodon.org/api/guidelines/'
+              %li= link_to t('about.api'), 'https://docs.joinmastodon.org/client/intro/'
           .column-2
             %h4= link_to t('about.what_is_mastodon'), 'https://joinmastodon.org/'
             = link_to svg_logo, root_url, class: 'brand'