To put a backtick in a markdown code block, i.e. within backticks, you just add more backticks around your code block.
Really this is more useful in the inline code blocks, for instance I can do console.log(`you're on the ${branchName} branch`).
So inline instead of wrapping with a single ` backtick, you wrap with two backticks, like ``console.log(`this ${demo}`)`` (and to get two backticks in the example shown, I needed to wrap with three backticks).
One final tip, to print a single backtick, it needs two backticks around it, but also a space around the single backtick too: `` ` ``.
Basically, it's backtick-inception.