Update link to migration guide (#107)

Author: mcmorisi

source/upgrade.txt

@@ -27,7 +27,7 @@ when you upgrade to a new version of {+driver-short+}.
 
    This guide includes breaking changes to only {+driver-short+} versions after v4.0.
    If you're upgrading from {+driver-short+} v2 or v3, see the
-   `{+driver-short+} 4 Migration Guide. <https://pymongo.readthedocs.io/en/latest/migrate-to-pymongo4.html>`__
+   `{+driver-short+} 4 Migration Guide. <https://pymongo.readthedocs.io/en/4.8.0/migrate-to-pymongo4.html>`__
 
 Before you upgrade, perform the following actions: