Update Add to Slack link

Update to remove GA link tracking that seems to be causing it to fail

docs/user/slack.md

@@ -9,7 +9,7 @@ Click the "Add to Slack" button below to invite the Atomist bot into
 your Slack workspace.
 
 <div style="text-align:center;">
-  <a href="https://atm.st/2wiDlUe" onclick="trackOutboundLink('https://atm.st/2wiDlUe'); return false;" target="_blank">
+  <a href="https://atm.st/2wiDlUe" target="_blank">
     <img alt="Add to Slack" height="50" width="174" src="https://platform.slack-edge.com/img/add_to_slack.png" srcset="https://platform.slack-edge.com/img/add_to_slack.png 1x, https://platform.slack-edge.com/img/add_to_slack@2x.png 2x" />
   </a>
 </div>
@@ -123,4 +123,4 @@ Sometimes all parameters are optional, and the only way to override them is on t
 For instance: `@atomist reset goals branch=my-branch` will trigger goal-setting on your branch instead
 of on the master branch.
 
-To see all the parameters, use `@atomist describe skill <skill>`.
+To see all the parameters, use `@atomist describe skill <skill>`.