Add migration line for start_span no span arg

Author: sl0thentr0py

MIGRATION_GUIDE.md

@@ -11,6 +11,7 @@ Looking to upgrade from Sentry SDK 2.x to 3.x? Here's a comprehensive list of wh
 
 - The SDK now supports Python 3.7 and higher.
 - `sentry_sdk.start_span` now only takes keyword arguments.
+- `sentry_sdk.start_span` no longer takes an explicit `span` argument.
 - The `Span()` constructor does not accept a `hub` parameter anymore.
 - `Span.finish()` does not accept a `hub` parameter anymore.
 - The `Profile()` constructor does not accept a `hub` parameter anymore.