fix typo

Author: chaozg

cuqi/problem/_problem.py

@@ -614,7 +614,7 @@ def _samplepCN(self, Ns, Nb, callback=None, experimental=False):
 
             scale = 0.02
 
-            sampler = cuqi.experimental.mcmc.pCN(self.posterior, scale, callback=callback)
+            sampler = cuqi.experimental.mcmc.PCN(self.posterior, scale, callback=callback)
 
             ti = time.time()