Add a period to the end of step

[nguyenalex836]

Code of Conduct

• [X] I have read and agree to the GitHub Docs project's Code of Conduct

What article on docs.github.com is affected?

https://docs.github.com/en/get-started/start-your-journey/hello-world#step-4-open-a-pull-request

What part(s) of the article would you like to see updated?

Click New pull request

Add a period to the end of this step to align with other steps (and style guide)

Additional information

Style guide reference