Dropbox source connector: add video link for refresh tokens; update curl command example for refresh tokens (#630)

Author: Paul-Cornell

snippets/general-shared-text/dropbox.mdx

@@ -66,6 +66,16 @@ Dropbox app access tokens are valid for **only four hours**. After this time, yo
 
 To have Unstructured automatically replace expired access tokens on your behalf, do the following:
 
+<iframe
+width="560"
+height="315"
+src="https://www.youtube.com/embed/PZyRgpPNEUs"
+title="YouTube video player"
+frameborder="0"
+allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture"
+allowfullscreen
+></iframe>
+
 1. Get the app key and app secret values for your Dropbox app. To do this:
 
    a) Sign in to the [Dropbox Developers](https://www.dropbox.com/developers) portal with the same credentials as your Dropbox account.<br/>
@@ -86,15 +96,16 @@ To have Unstructured automatically replace expired access tokens on your behalf,
 6. Use the [curl](https://curl.se/) utility in your Terminal or Command Prompt, or use a REST API client such as 
    [Postman](https://www.postman.com/product/api-client/), to make the following REST API call, replacing the following placeholders:
 
-   - Replace `<access-code>` with the access code that you just copied.
    - Replace `<app-key>` with the app key for your Dropbox app.
    - Replace `<app-secret>` with the app secret for your Dropbox app.
+   - Replace `<access-code>` with the access code that you just copied.
 
    ```text
-   curl https://api.dropbox.com/oauth2/token \
-   --data code=<access-code> \
-   --data grant_type=authorization_code \
-   --user <app-key>:<app-secret>
+   curl --location --request POST 'https://api.dropbox.com/oauth2/token' \
+   --user '<app-key>:<app-secret>' \
+   --header 'Content-Type: application/x-www-form-urlencoded' \
+   --data-urlencode 'code=<access-code>' \
+   --data-urlencode 'grant_type=authorization_code' \
    ```
 
 7. In the response, copy the following two values: