Update README.md (ecmwf#233)

Fixed typos

Author: ecwiebe

README.md

@@ -6,7 +6,7 @@
   <h1>The WeatherGenerator <br> Machine Learning Earth System Model</h1>
 </div>
 
-The WeatherGenerator project develops a machine learning-based Earth system models. 
+The WeatherGenerator project is developing a machine learning-based Earth system model. 
 It will be trained on a wide range of datasets, including reanalyses, forecast data and observations, to provide a robust and versatile model for the dynamics.
 Through this, it can be used for a wide-range of applications.