Fix broken links

Author: mattcone

_tools/buttondown.md

@@ -65,7 +65,7 @@ syntax:
     notes: "Some HTML is sanitized."
 ---
 
-[Buttondown](https://buttondown.com) is an email platform, similar to the now-defunct [TinyLetter](https://buttondown.com/comparisons/tinyletter), that allows you to send emails to subscribers. It also has integrations with many other platforms to automatically publish or add subscribers, amongst other possibilities.
+[Buttondown](https://buttondown.com) is an email platform that allows you to send emails to subscribers. It also has integrations with many other platforms to automatically publish or add subscribers, amongst other possibilities.
 
 The email editor [supports Markdown](https://buttondown.com/features/markdown), with the possibility to switch to a WYSIWYG editor for those who prefer it.
 

_tools/slack.md

@@ -351,6 +351,5 @@ The Slack post interface provides support for the following Markdown elements.
 ### See Also
 
 - [Formatting Slack messages in the interface](https://get.slack.help/hc/en-us/articles/202288908-Format-your-messages)
-- [Formatting Slack posts in the interface](https://get.slack.help/hc/en-us/articles/203950418-Composing-a-Post#-formatting-options)
 - [API documentation for formatting Slack messages](https://api.slack.com/messaging/composing/formatting)
 - [slack_markdown ruby gem](https://github.com/rutan/slack_markdown)