Corrected typos in the references (#713)

* Corrected typos in the references

Signed-off-by: star1327p <star1327p@gmail.com>

Author: star1327p

README.md

@@ -739,20 +739,20 @@ We invite you to join us at regular office hours and community calls in the Disc
 
 Athey, Susan, and Stefan Wager.
 **Policy learning with observational data.**
-Econometrica 89.1 (2021): 133-161.
+[*Econometrica 89.1, 133-161*](https://doi.org/10.3982/ECTA15732), 2021.
 
 X Nie, S Wager.
 **Quasi-Oracle Estimation of Heterogeneous Treatment Effects.**
-[*Biometrika*](https://doi.org/10.1093/biomet/asaa076), 2020
+[*Biometrika 108.2, 299-319*](https://doi.org/10.1093/biomet/asaa076), 2021.
 
 V. Syrgkanis, V. Lei, M. Oprescu, M. Hei, K. Battocchi, G. Lewis.
 **Machine Learning Estimation of Heterogeneous Treatment Effects with Instruments.**
-[*Proceedings of the 33rd Conference on Neural Information Processing Systems (NeurIPS)*](https://arxiv.org/abs/1905.10176), 2019
+[*Proceedings of the 33rd Conference on Neural Information Processing Systems (NeurIPS)*](https://arxiv.org/abs/1905.10176), 2019.
 **(Spotlight Presentation)**
 
 D. Foster, V. Syrgkanis.
 **Orthogonal Statistical Learning.**
-[*Proceedings of the 32nd Annual Conference on Learning Theory (COLT)*](https://arxiv.org/pdf/1901.09036.pdf), 2019
+[*Proceedings of the 32nd Annual Conference on Learning Theory (COLT)*](https://arxiv.org/pdf/1901.09036.pdf), 2019.
 **(Best Paper Award)**
 
 M. Oprescu, V. Syrgkanis and Z. S. Wu.
@@ -781,4 +781,4 @@ V. Chernozhukov, D. Chetverikov, M. Demirer, E. Duflo, C. Hansen, and a. W. Newe
 
 Dudik, M., Erhan, D., Langford, J., & Li, L.
 **Doubly robust policy evaluation and optimization.**
-Statistical Science, 29(4), 485-511, 2014.
+[*Statistical Science, 29(4), 485-511*](https://projecteuclid.org/journals/statistical-science/volume-29/issue-4/Doubly-Robust-Policy-Evaluation-and-Optimization/10.1214/14-STS500.full), 2014.