Docs: improve the information about the user name

Author: ForNeVeR

README.md

@@ -23,9 +23,11 @@ To use GoogleTaskExport, you'll have to register an application with Google and
 Use the following shell command:
 
 ```console
-$ dotnet run --project GoogleTaskExport -- [json|print] <username> <path to the JSON credential file>
+$ dotnet run --project GoogleTaskExport -- [json|print] <userName> <path to the JSON credential file>
 ```
 
+Here, `<userName>` is the user email, e.g. `example@gmail.com`.
+
 This command will fetch all the stored Google Tasks (including the completed ones, but omitting the deleted ones) and output them in the specified format. 
 
 Note that on the first run, it will open a browser window to ask you to grant the application access to your Google Tasks data.