[5.0.x] Added missing import in docs/ref/models/expressions.txt.

Backport of e28bd6776dfedc1ad4dce4719e4303ca34019a83 from main

docs/ref/models/expressions.txt

@@ -92,7 +92,7 @@ Some examples
     >>> Company.objects.order_by("name__length")
 
     # Boolean expression can be used directly in filters.
-    >>> from django.db.models import Exists
+    >>> from django.db.models import Exists, OuterRef
     >>> Company.objects.filter(
     ...     Exists(Employee.objects.filter(company=OuterRef("pk"), salary__gt=10))
     ... )