Update README.md

README.md

@@ -1,6 +1,6 @@
 # grmpy
 
-``grmpy``  is an open-source Python package for the simulation and estimation of generalized Roy Model. Its main purpose is to serve as a teaching tool to promote the conceptual framework provided by the generalized Roy model to illustrate a variety of issues in the econometrics of policy evaluation. The main references are:
+``grmpy``  is an open-source Python package for the simulation and estimation of generalized Roy model. Its main purpose is to serve as a teaching tool to promote the conceptual framework provided by the generalized Roy model to illustrate a variety of issues in the econometrics of policy evaluation. The main references are:
 
 *  Heckman, J. J., and Vytlacil, E. J. (2007a). [Econometric evaluation of social programs, Part I: Causal effects, structural models and econometric policy evaluation.](http://ac.els-cdn.com/S1573441207060709/1-s2.0-S1573441207060709-main.pdf?_tid=b933f5c8-6bbe-11e7-8ae8-00000aacb35d&acdnat=1500385435_c69182d36b79b66bbce5f5a7c593617c) In Heckman, J. J., and Leamer, E. E., editors, *Handbook of Econometrics*, volume 6B, pages 4779–4874. Elsevier
 Science, Amsterdam, Netherlands.