webhooks/basecamp: Update text and remove screenshots.

This commit:

* Removes the unnecessary screenshot. The UI is intuitive enough
  and standalone instructions should suffice.
* Rearranges the instructions into 4 steps.
* Makes the wording more explicit.

zerver/webhooks/basecamp/doc.md

@@ -5,18 +5,14 @@ events in Basecamp.
 
 1. {!create-bot-construct-url-indented.md!}
 
-1. Go to your project on Basecamp and choose **Set up webhooks**
-   from the **Settings** menu in the top right corner. Click on
+1. Go to your project on Basecamp. From the **Settings** menu in
+   the top right corner, choose **Set up webhooks**. Click on
    the **Add a new webhook** button.
 
-1. Paste the URL you created above and select the events you would like to
-   be notified about. Make sure the checkbox under **Enable this webhook?**
-   is checked.
-
-    ![](/static/images/integrations/basecamp/001.png)
-
-1. Click **Add this webhook**.
+1. Set **Payload URL** to the URL you created above. Select the events
+   you would like to be notified about, and check the checkbox under
+   **Enable this webhook**. Click **Add this webhook**.
 
 {!congrats.md!}
 
-![](/static/images/integrations/basecamp/002.png)
+![](/static/images/integrations/basecamp/001.png)