update jspm installation instructions

[aluanhaddad]

This updates the jspm installation instructions to remove npm: registry prefix as it is no longer necessary as the package is now mapped in the jspm registry.

Additionally. the example tsconfig.json now reflects the path corresponding to the latest package version and uses the lowercase module kind name "system" to match the other example and to avoid warnings issued by VS Code when "System" is specified.

[msftclas]

@aluanhaddad, Thanks for having already signed the Contribution License Agreement. Your agreement was validated by Microsoft. We will now review your pull request. Thanks, Microsoft Pull Request Bot

[mhegazy]

Thanks!