docs(slack): externalId -> external_id

Author: 3v0k4

docs/docs/slack/custom-apps.mdx

@@ -97,7 +97,7 @@ To send a DM in Slack you can use the user's email address.
     "recipients": [
       {
         "email": "dan@example.com",
-        "externalId": "AB1234BCA"
+        "external_id": "AB1234BCA"
       }
     ],
     "custom_attributes": {
@@ -120,7 +120,7 @@ curl https://api.magicbell.com/notifications \
       "title": "Welcome to Example Inc",
       "recipients": [{
         "email": "dan@example.com",
-        "externalId": "AB1234BCA"
+        "external_id": "AB1234BCA"
       }],
       "custom_attributes": {
         "name": "Dan",