Fix doctype reference

Author: MattiSG

openfisca_core/variables/variable.py

@@ -309,7 +309,7 @@ def get_formula(self, period = None):
         If no period is given and the variable has several formula, return the oldest formula.
 
         :returns: Formula used to compute the variable
-        :rtype: Formula
+        :rtype: :any:`Formula`
 
         """