fix tutorial image typo

[williamjuan027]

Fixes #70

[cla-bot]

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[cla-bot]

Thank you for your pull request and welcome to our community. We could not parse the GitHub identity of the following contributors: NativeScript-Bot.
This is most likely caused by a git client misconfiguration; please make sure to:

1. check if your git client is configured with an email to sign commits git config --list | grep email
2. If not, set it up using git config --global user.email email@example.com
3. Make sure that the git commit email is configured in your GitHub account settings, see https://github.com/settings/emails