Fix code example

Author: ricardoV94

pymc_experimental/distributions/continuous.py

@@ -261,9 +261,11 @@ class Chi:
     Examples
     --------
     .. code-block:: python
+        import pymc as pm
+        from pymc_experimental.distributions import Chi
 
         with pm.Model():
-            x = pm.Chi('x', nu=1)
+            x = Chi('x', nu=1)
     """
 
     @staticmethod