Fix readme typo

Author: marcharper

README.md

@@ -418,7 +418,7 @@ features, and add unit tests! Use the pull request and issue systems to contribu
 Please cite as follows:
 
 ```
-Marc Harper et al.. (2015). python-ternary: Ternary Plots in Python. Zenodo. 10.5281/zenodo.34938
+Marc Harper et al. (2015). python-ternary: Ternary Plots in Python. Zenodo. 10.5281/zenodo.34938
 ```
 
 # Recent Usage in Publications: