Resolve #463 (Prior Constraints "Simple Example" not working)

This section in the PriorDict sampling seems to be unnecessary since Constraints are checked at many other points in this process. Removing this section fixes the example that was broken by this in #463 (closed)

docs/prior.txt

@@ -178,8 +178,9 @@ First thing: define a function which generates z=x-y from x and y.
         -------
         dict: Dictionary with constraint parameter 'z' added.
         """
-        parameters['z'] = parameters['x'] - parameters['y']
-        return parameters
+        converted_parameters = parameters.copy()
+        converted_parameters['z'] = parameters['x'] - parameters['y']
+        return converted_parameters
 
 Create our prior: