Fix async example in session.execute() deprecation warning & docstring

Author: DanielLeviLucas

sqlmodel/ext/asyncio/session.py

@@ -119,13 +119,15 @@ async def exec(
         For example:
 
         ```Python
-        heroes = await session.execute(select(Hero)).scalars().all()
+        result = await session.execute(select(Hero))
+        heroes = result.scalars().all()
         ```
 
         instead you could use `exec()`:
 
         ```Python
-        heroes = await session.exec(select(Hero)).all()
+        result = await session.exec(select(Hero))
+        heroes = result.all()
         ```
         """
     )
@@ -148,13 +150,15 @@ async def execute(
         For example:
 
         ```Python
-        heroes = await session.execute(select(Hero)).scalars().all()
+        result = await session.execute(select(Hero))
+        heroes = result.scalars().all()
         ```
 
         instead you could use `exec()`:
 
         ```Python
-        heroes = await session.exec(select(Hero)).all()
+        result = await session.exec(select(Hero))
+        heroes = result.all()
         ```
         """
         return await super().execute(