docs: assign multicall to "m" in code example

This example snippet uses "m" without assigning. Update the code snippet
to match the other examples where we assign "m" in the context manager.

docs/source/writing_koji_code.rst

@@ -247,7 +247,7 @@ or they may be passed to the ``call_all`` method.
 
 ::
 
-    with session.multicall(strict=True, batch=500):
+    with session.multicall(strict=True, batch=500) as m:
         builds = [m.getBuild(build_id) for build_id in mylist]