[Examples] Fix reservoir synchronization in surf_pfr_chain.py (Cantera#2069)

Author: speth

samples/python/reactors/surf_pfr_chain.py

@@ -105,7 +105,7 @@
 
 for n in range(NReactors):
     # Set the state of the reservoir to match that of the previous reactor
-    gas.TDY = r.phase.TDY
+    upstream.phase.TDY = r.phase.TDY
     upstream.syncState()
     sim.reinitialize()
     sim.advance_to_steady_state()