Update "Specifying the theme an image is shown to" section in basic-writing-and-formatting-syntax.md

[nguyenalex836]

Code of Conduct

• 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/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#specifying-the-theme-an-image-is-shown-to

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

The following changes need to be made to this section of this docs:

• Rename it to just state "The Picture element" (similar to how Mozilla styles it)

• Remove all the current content and replace it with a sentence that states "The <picture> HTML element is supported."

Additional information

No response