Adjusted command line arguments of code generators

The `-o` argument is required now.

Author: asishallab

README.md

@@ -136,14 +136,14 @@ generators and/or skeleton server projects `graphql-server` or
 
 ```
 docker run --rm -it -v `pwd`:/opt --user 1000:1000 sciencedb-code-generators:latest 
-graphql-server-model-codegen generate --jsonFiles /opt/data_model_definitions /opt/graphql-server
+graphql-server-model-codegen generate --jsonFiles /opt/data_model_definitions -o /opt/graphql-server
 ```
 
 #### Generate the Single Page Application (SPA) server
 
 ```
 docker run --rm -it -v `pwd`:/opt --user 1000:1000 sciencedb-code-generators:latest 
-single-page-app-codegen --jsonFiles /opt/data_model_definitions /opt/single-page-app
+single-page-app-codegen --jsonFiles /opt/data_model_definitions -o /opt/single-page-app
 ```
 
 ### Multiple code generation