Update README.md

as per warning in command line:

```
Warning: use of 'dotnet new --install' is deprecated. Use 'dotnet new install' instead.
```

Author: mcraveiro

README.md

@@ -30,7 +30,7 @@ Thanks to its modularity and extensibility, SimpleIdServer can be customized to
 Install SimpleIdServer templates.
 
 ```
-dotnet new --install SimpleIdServer.Templates
+dotnet new install SimpleIdServer.Templates
 ```
 
 This will add the following templates