fix: purchase -> creation

Author: bryantanderson

fern/server-url/setting-server-urls.mdx

@@ -39,7 +39,7 @@ Here's a breakdown of where you can set server URLs in Vapi:
     Phone numbers can have a server URL attached to them via the [phone number API](/api-reference/phone-numbers).
 
     The server URL for phone numbers can be set **3 ways**:
-    - **At Time of Purchase:** when you [create a number](/api-reference/phone-numbers/create) through Vapi
+    - **At Time of Creation:** when you [create a free number](/api-reference/phone-numbers/create) through Vapi
     - **At Import:** when you [import from Twilio](/api-reference/phone-numbers/import-twilio-number) or [Vonage](/api-reference/phone-numbers/import-vonage-number)
     - **Via Update:** you can [update a number](/api-reference/phone-numbers/update-phone-number) already in your account