Make it more understandable

[KennethKinLum]

The original text could be quite confusing:

Open your ~/.ssh/config file, then modify the file, replacing ~/.ssh/id_ed25519 if you are not using the default location and name for your id_ed25519 key.

Host *
  AddKeysToAgent yes
  UseKeychain yes
  IdentityFile ~/.ssh/id_ed25519

1. It said "Open your ~/.ssh/config file, then modify the file, replacing ~/.ssh/id_ed25519" -- I was thinking, open that file, and then modify it, BUT HOW? AND THEN, replacing the file content of ~/.ssh/id_ed25519 ? By what content? It turned out it meant, modify it to contain the following lines, and for the path and filename ~/.ssh/id_ed25519 on the last line, use the proper one if you did not use the default one.
2. And then it said, "if you are not using the default location and name for your id_ed25519 key"... location and name? What location and name? It turned out it meant the "path" and the "filename"... and "if you are using"? It actually meant "if you used" in the previous steps.

Why:

Closes [issue link]

What's being changed:

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• I have reviewed my changes in staging. (look for the deploy-to-heroku link in your pull request, then click View deployment)
• For content changes, I have reviewed the localization checklist
• For content changes, I have reviewed the Content style guide for GitHub Docs.

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[janiceilene]

@KennethKinLum Thanks so much for opening a PR! I'll get this triaged for review ⚡

[ethanpalm]

👋🏼 @KennethKinLum, thanks for this PR! I agree that this language is quite confusing and I appreciate you taking the time to improve it. I made one small change just to keep the language consistent with some other GitHub docs. I'll merge this PR as soon as all the checks are passing. The changes will be live within 24 hours.

You have a great analysis of the problem and identified a nice solution. Please check out the help wanted issues if you are interested in contributing again!

Thanks for helping make our docs better! ✨ 🚀

[ethanpalm]

Suggested change

	* Open your `~/.ssh/config` file, then modify the file to contain the following lines. Replace the path and filename ` ~/.ssh/id_ed25519` below if you did not use the default path and filename for your `id_ed25519` key.
	* Open your `~/.ssh/config` file, then modify the file to contain the following lines. If your SSH key file has a different name or path than the example code, modify the filename or path to match your current setup.

This change makes the step similar to the procedure in "Adding a new SSH key to your GitHub account." Since not everyone will have to do this step, I think starting the sentence with "If your SSH key file has a different name or path" can help people know a bit more quickly if they need to do the step or not.

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